0001564590-21-010057 | 10-K | XBRL Viewer

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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2020	Feb. 24, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Trading Symbol	MCRB
Entity Registrant Name	Seres Therapeutics, Inc.
Entity Central Index Key	0001609809
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity File Number	001-37465
Entity Tax Identification Number	27-4326290
Entity Address, Address Line One	200 Sidney Street – 4th Floor
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	945-9626
Entity Public Float			$ 219,853,799
Entity Common Stock Shares Outstanding		91,549,412
Entity Incorporation, State or Country Code	DE
Document Annual Report	true
Document Transition Report	false
Entity Well Known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common stock, par value $0.001 per share
Security Exchange Name	NASDAQ
Documents Incorporated by Reference	Portions of the registrant’s definitive Proxy Statement relating to its 2021 Annual Meeting of Stockholders to be filed with the SEC within 120 days after the end of the fiscal year ended December 31, 2020 are incorporated herein by reference in Part III.

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 116,049	$ 65,126
Short term investments	137,567	29,690
Prepaid expenses and other current assets	5,774	3,588
Accounts receivable	9,387	1,785
Total current assets	268,777	100,189
Property and equipment, net	13,897	19,495
Operating lease assets	9,041	11,356
Restricted investments	1,400	1,400
Long term investments	49,825
Total assets	342,940	132,440
Current liabilities:
Accounts payable	4,018	4,859
Accrued expenses and other current liabilities	14,226	10,884
Operating lease liabilities	5,115	4,456
Short term portion of note payable, net of discount	454
Deferred revenue - related party	22,602	20,960
Deferred revenue		4,834
Total current liabilities	46,415	45,993
Long term portion of note payable, net of discount	24,639	24,648
Operating lease liabilities, net of current portion	10,561	15,676
Deferred revenue, net of current portion - related party	85,572	89,111
Deferred revenue, net of current portion		4,834
Other long-term liabilities	1,003	502
Total liabilities	168,190	180,764
Commitments and contingencies (Note 14)
Stockholders’ equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2020 and 2019; no shares issued and outstanding at December 31, 2020 and 2019
Common stock, $0.001 par value; 200,000,000 shares authorized at December 31, 2020 and 2019; 91,459,239 and 70,143,252 shares issued and outstanding at December 31, 2020 and 2019	91	70
Additional paid-in capital	723,482	411,255
Accumulated other comprehensive loss	(47)
Accumulated deficit	(548,776)	(459,649)
Total stockholders’ equity (deficit)	174,750	(48,324)
Total liabilities and stockholders’ equity (deficit)	$ 342,940	$ 132,440

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	91,459,239	70,143,252
Common stock, shares outstanding	91,459,239	70,143,252

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue:
Collaboration revenue - related party	$ 11,897,000	$ 27,188,000	$ 26,917,000
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Grant revenue	$ 4,157,000	$ 1,102,000	$ 1,350,000
Collaboration revenue	17,161,000	6,215,000
Total revenue	33,215,000	34,505,000	28,267,000
Operating expenses:
Research and development expenses	90,570,000	80,141,000	95,955,000
General and administrative expenses	30,775,000	24,748,000	32,596,000
Restructuring expenses	0	1,492,000	0
Total operating expenses	121,345,000	106,381,000	128,551,000
Loss from operations	(88,130,000)	(71,876,000)	(100,284,000)
Other (expense) income:
Interest income	946,000	1,033,000	1,172,000
Interest expense	(2,924,000)	(502,000)
Other income	981,000	1,066,000	170,000
Total other (expense) income, net	(997,000)	1,597,000	1,342,000
Net loss	$ (89,127,000)	$ (70,279,000)	$ (98,942,000)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.12)	$ (1.24)	$ (2.43)
Weighted average common shares outstanding, basic and diluted	79,789,220	56,649,220	40,743,492
Other comprehensive (loss) income:
Unrealized (loss) gain on investments, net of tax of $0	$ (47,000)		$ 146,000
Total other comprehensive income (loss)	(47,000)		146,000
Comprehensive loss	$ (89,174,000)	$ (70,279,000)	$ (98,796,000)

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Unrealized (loss) gain on investment, tax	$ 0	$ 0	$ 0

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Accumulated Deficit [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]
Beginning balance at Dec. 31, 2017	$ 60,699		$ 40	$ 324,376	$ (146)	$ (263,571)
Beginning balance, shares at Dec. 31, 2017			40,571,015
Issuance of common stock upon exercise of stock options, value	147		$ 1	146
Issuance of common stock upon exercise of stock options, shares			212,240
Issuance of common stock upon vesting of RSUs, net of tax withholdings, value	61			61
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares			138,048
Repurchase of common stock for employee tax withholdings, value	(197)			(197)
Repurchase of common stock for employee tax withholdings, shares			(17,900)
Issuance of common stock under ESPP plan, value	257			257
Issuance of common stock under ESPP plan, shares			33,332
Stock-based compensation expense	16,641			16,641
Unrealized gain (loss) on investments	146				146
Net loss	(98,942)					(98,942)
Ending balance at Dec. 31, 2018	(48,045)	$ (26,857)	$ 41	341,284		(389,370)	$ (26,857)
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201409Member					us-gaap:AccountingStandardsUpdate201409Member
Ending balance, shares at Dec. 31, 2018			40,936,735
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, value	60,527		$ 29	60,498
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares			28,818,578
Issuance of common stock from at the market equity offering, value	512			512
Issuance of common stock from at the market equity offering, shares			128,400
Issuance of common stock upon exercise of stock options, value	145			145
Issuance of common stock upon exercise of stock options, shares			90,125
Issuance of common stock upon vesting of RSUs, net of tax withholdings, value	176			176
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares			94,400
Issuance of common stock under ESPP plan, value	296			296
Issuance of common stock under ESPP plan, shares			75,014
Stock-based compensation expense	8,344			8,344
Net loss	(70,279)					(70,279)
Ending balance at Dec. 31, 2019	$ (48,324)		$ 70	411,255		(459,649)
Ending balance, shares at Dec. 31, 2019	70,143,252		70,143,252
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, value	$ 243,748		$ 12	243,736
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares			12,075,000
Issuance of common stock from Securities Purchase Agreement, net of offering costs - related party, value	19,900		$ 1	19,899
Issuance of common stock from Securities Purchase Agreement, net of offering costs - related party, shares			959,002
Issuance of common stock from at the market equity offering, value	24,773		$ 6	24,767
Issuance of common stock from at the market equity offering, shares			5,787,681
Issuance of common stock upon exercise of stock options, value	$ 14,421		$ 2	14,419
Issuance of common stock upon exercise of stock options, shares	2,214,011		2,214,011
Issuance of common stock upon vesting of RSUs, net of tax withholdings, value	$ 120			120
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares			125,000
Issuance of common stock under ESPP plan, value	462			462
Issuance of common stock under ESPP plan, shares			155,293
Stock-based compensation expense	8,824			8,824
Unrealized gain (loss) on investments	(47)				(47)
Net loss	(89,127)					(89,127)
Ending balance at Dec. 31, 2020	$ 174,750		$ 91	$ 723,482	$ (47)	$ (548,776)
Ending balance, shares at Dec. 31, 2020	91,459,239		91,459,239

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (89,127)	$ (70,279)	$ (98,942)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	8,824	8,344	16,641
Depreciation and amortization expense	6,578	7,603	7,862
Non-cash operating lease cost	2,315	2,227
Amortization of debt issuance costs	446	281
Accretion (amortization) of discount (premium) on issued debt securities	551	(172)	(214)
Loss on disposal of property and equipment		103
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,186)	3,257	(2,056)
Accounts receivable	(7,602)	(1,785)
Deferred revenue	(11,565)	(17,520)	13,476
Accounts payable	(1,159)	(1,460)	(353)
Operating lease liabilities	(4,456)	(4,211)
Accrued expenses and other liabilities	3,771	(2,908)	732
Net cash (used in) operating activities	(93,610)	(76,520)	(62,854)
Cash flows from investing activities:
Purchases of property and equipment	(591)	(1,002)	(1,937)
Purchases of investments	(218,284)	(46,420)	(21,832)
Sales and maturities of investments	59,984	16,904	136,087
Net cash (used in) provided by investing activities	(158,891)	(30,518)	112,318
Cash flows from financing activities:
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs	243,748	60,527
Proceeds from Securities Purchase Agreement, net of issuance costs - related party	19,900
Proceeds from issuance of note payable		25,000
Proceeds from at the market equity offering, net of commissions	24,773	512
Payments of debt issuance costs		(425)
Proceeds from exercise of stock options	14,421	145	147
Proceeds from issuance of common stock and restricted common stock	120	176	61
Payments for repurchase of common stock for employee tax withholdings			(197)
Issuance of common stock under ESPP plan	462	296	257
Net cash provided by financing activities	303,424	86,231	268
Net increase (decrease) in cash, cash equivalents and restricted cash	50,923	(20,807)	49,732
Cash, cash equivalents and restricted cash at beginning of year	65,126	85,933	36,201
Cash, cash equivalents and restricted cash at end of year	116,049	65,126	85,933
Supplemental disclosure of cash flow information:
Cash paid for interest	2,453	221
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 451	62	$ 157
Reduction of operating lease assets and operating lease liabilities from operating lease modifications or reassessments		$ 154

Nature of the Business and Basis of Presentation

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Nature of the Business and Basis of Presentation

1.	Nature of the Business and Basis of Presentation

Seres Therapeutics, Inc. (the “Company”) was incorporated under the laws of the State of Delaware in October 2010 under the name Newco LS21, Inc. In October 2011, the Company changed its name to Seres Health, Inc., and in May 2015, the Company changed its name to Seres Therapeutics, Inc. The Company is a microbiome therapeutics platform company developing a novel class of biological drugs, which are designed to treat disease by modulating the microbiome to restore health by repairing the function of a disrupted microbiome to a non-disease state. The Company’s lead product candidate, SER-109, is designed to prevent further recurrences of Clostridioides difficle infection (“CDI”), a debilitating infection of the colon, in patients who have received antibiotic therapy for recurrent CDI by treating the disruption of the colonic microbiome. If approved by the U.S. Food and Drug Administration (“FDA”), we believe SER-109 will be a first-in-field oral microbiome drug. SER-287 and SER-301 are being developed by the Company to treat ulcerative colitis (“UC”). In addition, using its microbiome therapeutics platform, the Company is also developing product candidates to treat diseases where the microbiome is implicated, including SER-155, a consortium of cultivated bacteria, therapeutics candidate designed to prevent mortality due to gastrointestinal infections, bacteremia and graft versus host disease (GvHD”) in immunocompromised patients, including in patients receiving allogeneic hematopoietic stem cell transplantation (“allo-HSCT”) and solid organ transplants and SER-401, a microbiome therapeutics candidate for use with checkpoint inhibitors (CPI’s”) in patients with metastatic melanoma. The Company continues to evaluate microbiome pharmacokinetic of SER-262 to treat an initial recurrence of CDI. The Company is also using its reverse translation microbiome therapeutics platforms to conduct research on various indications, including pathogen infection and antibiotic resistant bacteria, inflammatory and immune diseases, cancer, and metabolic diseases.

The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities.

The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants.

On August 10, 2020, the Company reported positive topline results from its pivotal Phase 3 ECOSPOR III study evaluating SER-109 for recurrent CDI. The Company is actively enrolling patients in its SER-109 open-label study, which also admits patients with a single recurrence of CDI, to expand the safety database to meet the FDA threshold of at least 300 patients.

On August 12, 2020, the Company completed an underwritten public offering in which it sold 10,500,000 shares of its common stock at a public offering price of $21.50 per share. In addition, the Company granted the underwriters a 30-day option to purchase up to an additional 1,575,000 shares of its common stock at the public offering price, less underwriting discounts and commissions, which the underwriters exercised in full. The Company received aggregate net proceeds from the offering of approximately $243,748 after deducting underwriting discounts and commissions and offering expenses payable by the Company.

On August 12, 2020, the Company entered into a Securities Purchase Agreement (the “Securities Agreement”) with Société des Produits Nestlé S.A. (“Nestlé”) for the sale of 959,002 shares of its common stock at a purchase price of $20.855 per share (the “concurrent placement”). The Company received aggregate net proceeds from the concurrent placement of approximately $19,900 after deducting offering expenses payable by the Company.

Under Accounting Standards Update (“ASU”) 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40) (“ASC 205-40”), the Company has the responsibility to evaluate whether conditions and/or events raise substantial doubt about its ability to meet its future financial obligations as they become due within one year after the date that the financial statements are issued. As required by ASC 205-40, this evaluation shall initially not take into consideration the potential mitigating effects of plans that have not been fully implemented as of the date the financial statements are issued.

As of December 31, 2020, the Company had an accumulated deficit of $548,776 and cash, cash equivalents and short- and long-term investments of $303,441. For the year ended December 31, 2020, the Company incurred a loss of $89,127 and used $93,610 of cash in operations. The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. The Company expects that its cash, cash equivalents and short and long-term investments as of December 31, 2020 of $303,441 will be sufficient to fund its operating expenses, capital expenditure requirements, and debt service obligations for at least the next 12-months from issuance of the financial statements. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations.

The Company is eligible to receive contingent milestone payments under its license and collaboration agreement with Société des Produits Nestlé S.A. (“Nestlé”), successor in interest to Nestec Ltd., an affiliate of Nestlé Health Science US Holdings, Inc. (“Nestlé Health Science”), both of which are significant stockholder of the Company, if certain development milestones are achieved. However, these milestones are uncertain and there is no assurance that the Company will receive any of them. Until such time, if ever, as the Company can generate substantial product revenue, the Company will finance its cash needs through a combination of public or private equity offerings, debt financings, governmental funding, collaborations, strategic partnerships or marketing, distribution or licensing arrangements with third parties. The Company may not be able to obtain funding on acceptable terms, or at all. If the Company is unable to raise additional funds as and when needed, it would have a negative impact on the Company’s financial condition, which may require the Company to delay, reduce or eliminate certain research and development activities and reduce or eliminate discretionary operating expenses, which could constrain the Company’s ability to pursue its business strategies.

The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its wholly owned subsidiaries after elimination of all intercompany accounts and transactions.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition and the accrual of research and development expenses. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.

The full extent to which the COVID-19 pandemic will directly or indirectly impact the Company’s business, results of operations and financial condition, including revenue, operating expenses, clinical trials and employee-related amounts, will depend on future developments that are highly uncertain, including new information that may emerge concerning COVID-19 and the actions taken to contain it or treat its impact and the economic impact on local, regional, national and international markets. We have made estimates of the impact of COVID-19 within our financial statements and there may be changes to those estimates in future periods. Actual results could differ from the Company’s estimates.

Cash Equivalents

The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market accounts, commercial paper and corporate bonds purchased with original maturities of less than 90 days from the date of purchase, are stated at fair value.

Investments

The Company classifies its available-for-sale marketable debt securities as current assets on the consolidated balance sheet if they mature within one year from the balance sheet date. Any available-for-sale marketable debt securities with maturities greater than one year from the balance sheet date are classified as long-term assets on the consolidated balance sheet.

The Company classifies all of its marketable debt securities as available-for-sale securities. Accordingly, these marketable debt securities are recorded at fair value and unrealized gains and losses are reported as a separate component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). The cost of securities sold is determined on a specific identification

basis, and realized gains and losses are included in other income (expense) within the consolidated statement of operations and comprehensive loss. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary”, the Company reduces the investment to fair value through a charge to the consolidated statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.

Restricted Investments

The Company held investments of $1,400 as of December 31, 2020 and December 31, 2019 in a separate restricted bank account as a security deposit for the lease of the Company’s facilities. The Company has classified these deposits as long-term restricted investments on its balance sheet.

Concentration of Credit Risk and of Significant Suppliers

Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. The Company has all cash, cash equivalents and investments balances at accredited financial institutions, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including preclinical and clinical testing. These programs could be adversely affected by a significant interruption in the supply of such drug substance products.

Fair Value Measurements

Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

•

Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above. The Company’s investments in certificates of deposit are carried at amortized cost, which approximates fair value. The carrying values of the Company’s accounts receivable, prepaid expense and other current assets, accounts payable and accrued expenses approximates their fair value due to the short-term nature of these assets and liabilities. The carrying value of the Company’s long-term debt approximates its fair value (a level 2 measurement) at each balance sheet date due to its variable interest rate, which approximates a market interest rate.

Property and Equipment

Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset. Laboratory equipment is depreciated over five years. Computer equipment and furniture and office equipment are depreciated over three years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets.

Research and Development Costs

Research and development costs are expensed as incurred. Research and development expenses include salaries, stock-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials.

Research Contract Costs and Accruals

The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

Patent Costs

All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.

Accounting for Stock-Based Compensation

The Company measures all stock options and other stock-based awards granted to employees, non-employees, and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options, restricted stock units and restricted stock awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. For stock options or restricted stock units issued with performance-based vesting conditions, the stock compensation expense related to these awards is recognized based on the grant date fair value when achievement of the performance condition is deemed probable.

The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified.

The Company accounts for forfeitures of stock-based awards as they occur rather than applying an estimated forfeiture rate to stock-based compensation expense.

The fair value of each stock option grant is estimated on the date of grant using the Black- Scholes option-pricing model. The Company lacks sufficient company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The Company uses the simplified method

prescribed by Securities and Exchange Commission Staff Accounting Bulletin No. 107, Share-Based Payment, to calculate the expected term of options granted to employees, non-employees and directors. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

Revenue Recognition

The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption.

Under ASC 606, the Company recognizes revenue using the cost-to-cost method over the remaining performance period as described in Note 12. The cumulative effect of applying ASC 606 as of the adoption date of January 1, 2018 of $26,857 was recorded as an adjustment to accumulated deficit in the statement of stockholders’ equity (deficit).

The Company recognizes revenue in accordance with the guidance under ASC 606, Revenue from Contracts with Customers. ASC 606 applies to all contracts with customers, except those contracts that are within the scope of other guidance, such as leases, insurance, and financial instruments. The Company enters into agreements that are within the scope of ASC 606, under which the Company licenses certain of the Company’s product candidates and performs research and development services in connection with such arrangements. The terms of these arrangements typically include payment of one or more of the following: nonrefundable up-front fees, reimbursement of research and development costs, development, clinical, regulatory and commercial sales milestone payments, and royalties on net sales of licensed products. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. When determining the timing and extent of revenue recognition for arrangements that the Company determines are within the scope of ASC 606, the Company performs the following five steps:

(i)

identify the contract(s) with a customer;

(ii)

identify the performance obligations in the contract;

(iii)

determine the transaction price;

(iv)

allocate the transaction price to the performance obligations in the contract; and

(v)

recognize revenue when (or as) the entity satisfies a performance obligation.

The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services transferred to the customer.

At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within the contract to determine whether each promised good or service is a performance obligation. The promised goods or services in the Company’s arrangements typically consist of a license to the Company’s intellectual property and/or research and development services. The Company may provide options to additional items in such arrangements, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. Performance obligations are promises in a contract to transfer a distinct good or service to the customer that (i) the customer can benefit from on its own or together with other readily available resources, and (ii) is separately identifiable from other promises in the contract. Goods or services that are not individually distinct performance obligations are combined with other promised goods or services until such combined group of promises meet the requirements of a performance obligation.

The Company determines the transaction price based on the amount of consideration the Company expects to receive for transferring the promised goods or services in the contract. Consideration may be fixed, variable, or a combination of both. At contract inception for arrangements that include variable consideration, the Company estimates the probability and extent of consideration it expects to receive under the contract utilizing either the most likely amount method or expected amount method, whichever best estimates the amount expected to be received. The Company then considers any constraints on the variable consideration and includes in the transaction price variable consideration to the extent it is deemed probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved.

The Company then allocates the transaction price to each performance obligation based on the relative standalone selling price and recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) control is transferred to the customer and the performance obligation is satisfied. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

The Company records amounts as accounts receivable when the right to consideration is deemed unconditional. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded for deferred revenue. As of December 31, 2020 and December 31, 2019, the Company had $1,186 and $406 of accounts receivable and $8,201 and $1,379 of unbilled accounts receivable, respectively.

The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. Incremental costs of obtaining a contract are expensed as and when incurred if the expected period over which the Company would have amortized the asset is one year or less, or the amount is immaterial.

Collaboration Revenue

Arrangements with collaborators may include licenses to intellectual property, research and development services, manufacturing services for clinical and commercial supply, and participation on joint steering committees. The Company evaluates the promised goods or services to determine which promises, or group of promises, represent performance obligations. In contemplation of whether a promised good or service meets the criteria required of a performance obligation, the Company considers the stage of development of the underlying intellectual property, the capabilities and expertise of the customer relative to the underlying intellectual property, and whether the promised goods or services are integral to or dependent on other promises in the contract. When accounting for an arrangement that contains multiple performance obligations, the Company must develop judgmental assumptions, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success to determine the stand-alone selling price for each performance obligation identified in the contract.

When the Company concludes that a contract should be accounted for as a combined performance obligation and recognized over-time, the Company must then determine the period over which revenue should be recognized and the method by which to measure revenue. The Company generally recognizes revenue using a cost-based input method.

Licenses of intellectual property

If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue associated with the bundled performance obligation. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of progress and related revenue recognition.

Milestone Payments

At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service, otherwise it will be allocated to all performance obligations of the arrangement based on the initial allocation.

The Company evaluates each milestone to determine when and how much of the milestone to include in the transaction price. The Company first estimates the amount of the milestone payment that the Company could receive using either the expected value or

the most likely amount approach. The Company primarily uses the most likely amount approach as that approach is generally most predictive for milestone payments with a binary outcome. Then, the Company considers whether any portion of that estimated amount is subject to the variable consideration constraint (that is, whether it is probable that a significant reversal of cumulative revenue would not occur upon resolution of the uncertainty). The Company updates the estimate of variable consideration included in the transaction price at each reporting date which includes updating the assessment of the likely amount of consideration and the application of the constraint to reflect current facts and circumstances.

Royalties

For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any revenue related to sales-based royalties or milestone payments based on the level of sales.

Manufacturing supply services

For arrangements that include a promise of supply of clinical or commercial product, the Company determines if the supply is a promise in the contract or a future obligation at the customer’s option. If determined to be a promise at inception of the contract, the Company evaluates the promise to determine whether it is a separate performance obligation or a component of a bundled performance obligation. If determined to be an option, the Company determines if the option provides a material right to the customer and if so, accounts for the option as a separate performance obligation. If determined to be an option but not a material right, the Company accounts for the option as a separate contract when the customer elects to exercise the option.

Grant Revenue

The Company generates revenue from government contracts that reimburse the Company for certain allowable costs for funded projects. For contracts with government agencies, when the Company has concluded that it is the principal in conducting the research and development expenses, and where the funding arrangement is considered central to the Company’s ongoing operations, the Company classifies the recognized funding received as revenue.

The Company has concluded to recognize funding received as revenue, rather than as a reduction of research and development expenses, because the Company is the principal in conducting the research and development activities and these contracts are central to its ongoing operations. Revenue is recognized as the qualifying expenses related to the contracts are incurred. Revenue recognized upon incurring qualifying expenses in advance of receipt of funding is recorded in the Company’s consolidated balance sheet as accounts receivable. The related costs incurred by the Company are included in research and development expense in the Company’s consolidated statements of operations and comprehensive loss.

Income Taxes

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense.

The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Segment Data

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing microbiome therapeutics to treat the modulation of the colonic microbiome. Revenue to date has been generated solely through the Company's agreements with its collaborators, all of which has been earned in the United States. All tangible assets are held in the United States.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2020, 2019 and 2018, other comprehensive income (loss) consisted of changes in unrealized gains (losses) from available-for-sale investments.

Net Loss per Share

Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options and unvested restricted stock. The Company applies the two-class method to calculate its basic and diluted net loss per share attributable to common stockholders. The two-class method is an earnings allocation formula that treats a participating security as having rights to earnings that otherwise would have been available to common stockholders. However, the two-class method does not impact the net loss per share of common stock as the Company was in a net loss position for each of the periods presented.

The Company’s convertible preferred stock contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Similarly, restricted stock awards entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding common shares at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.

Leases

The Company adopted ASC 842 on January 1, 2019 using the modified retrospective approach with no restatement of prior periods or cumulative adjustment to accumulated deficit. The Company determined if an arrangement is a lease at contract inception based on the facts and circumstances present in the arrangement. All the Company’s leases are classified as operating leases under the new leasing standard. The Company records operating lease assets and lease liabilities in its consolidated balance sheets. Operating lease assets represent the Company’s right to use an underlying asset for the lease term and operating lease liabilities represent the Company’s obligation to make lease payments arising from the leasing arrangement. Operating lease assets and operating lease liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As most of the Company’s leases do not provide an implicit rate, in determining the operating lease liabilities, the Company uses an estimate of its incremental borrowing rate based on the information available at commencement. Lease expense for lease payments is recognized on a straight-line basis over the lease term. Short-term leases, or leases that have a lease term of 12 months or less at commencement date, are excluded from this treatment and are recognized on a straight-line basis over the term of the lease.

Recently Adopted Accounting Pronouncements

In November 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”). This standard makes targeted improvements for collaborative arrangements as follows:

•

Clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606, Revenue from Contracts with Customers, when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in ASC 606 should be applied, including recognition, measurement, presentation and disclosure requirements;

•

Adds unit-of-account guidance to ASC 808, Collaborative Arrangements, to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606; and

•

Precludes a company from presenting transactions with collaborative participants that are not directly related to sales to third parties with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer.

This standard became effective for the Company on January 1, 2020 and did not have a material impact on the Company’s consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”). This standard modifies certain disclosure requirements on fair value measurements. This standard became effective for the Company on January 1, 2020 and did not have a material impact on the Company’s disclosures.

Recently Issued Accounting Pronouncements

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (‘‘ASU 2016-13’’), which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016-13 replaces the existing incurred loss impairment model with an expected loss model. It also eliminates the concept of other-than-temporary impairment and requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes may result in earlier recognition of credit losses. In November 2018, the FASB issued ASU No. 2018-19, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, which narrowed the scope and changed the effective date for non-public entities for ASU 2016-13. The FASB subsequently issued supplemental guidance within ASU No. 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief (‘‘ASU 2019-05’’). ASU 2019-05 provides an option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. For public entities that are Securities and Exchange Commission filers, excluding entities eligible to be smaller reporting companies, ASU 2016-13 is effective for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. For all other entities, ASU 2016-13 is effective for annual periods beginning after December 15, 2022, including interim periods within those fiscal years. Early adoption is permitted. This standard will be effective for the Company on January 1, 2023. The Company is currently evaluating the potential impact that this standard may have on its condensed consolidated financial statements and related disclosures.

Fair Value of Financial Assets and Liabilities

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

3.	Fair Value of Financial Assets and Liabilities

The following tables present the Company’s fair value hierarchy for its assets and liabilities that are measured at fair value on a recurring basis (in thousands):

	Fair Value Measurements as of December 31, 2020 Using:
	Level 1		Level 2		Level 3		Total
Cash Equivalents:
Money market funds	$	35,480	$	—	$	—	$	35,480
Commercial paper		—		10,313		—		10,313
Corporate bonds		—		2,014		—		2,014
Investments:
Commercial paper	$	—	$	12,343	$	—	$	12,343
Corporate bonds		—		68,289		—		68,289
Certificate of deposits		—		2,272		—		2,272
Government securities		—		104,488		—		104,488
	$	35,480	$	199,719	$	—	$	235,199

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Cash Equivalents:
Money market funds	$	25,510	$	—	$	—	$	25,510
Commercial paper		—		4,243		—		4,243
Corporate bonds		—		4,900		—		4,900
Investments:
Commercial paper	$	—	$	11,957			$	11,957
Corporate bonds		—		17,733				17,733
	$	25,510	$	38,833	$	—	$	64,343

Money market funds were valued by the Company based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. Commercial paper, government securities, certificates of deposit and corporate bonds were valued by the Company using quoted prices in active markets for similar securities, which represent a Level 2 measurement within the fair value hierarchy. There were no transfers between Level 1 or Level 2 during the years ended December 31, 2020 and 2019.

As of December 31, 2020 and 2019 the Company held a restricted investment of $1,400, which represents a certificate of deposit that is classified as Level 2 in the fair value hierarchy.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Investments

12 Months Ended

Dec. 31, 2020

Investments Debt And Equity Securities [Abstract]

Investments

4.	Investments

Investments by security type consisted of the following at December 31, 2020 and December 31, 2019 (in thousands):

	December 31, 2020
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Commercial paper	$	12,343	$	—	$	—		$	12,343
Corporate bonds		68,333		8		(52	)		68,289
Certificate of deposits		2,272		-		-			2,272
Government securities		104,491		6		(9	)		104,488
	$	187,439	$	14	$	(61	)	$	187,392

	December 31, 2019
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Commercial paper	$	11,957	$	—	$	—		$	11,957
Corporate bonds		17,732		3		(2	)		17,733
	$	29,689	$	3	$	(2	)	$	29,690

Investments with original maturities of less than 90 days are included in cash and cash equivalents on the consolidated balance sheets and are not included in the table above. Investments with maturities of less than twelve months are considered current assets and those investments with maturities greater than twelve months are considered non-current assets.

Excluded from the table above is a restricted investment of $1,400 as the cost approximates current fair value.

The amortized cost and fair value of investments in commercial paper, corporate bonds, certificates of deposit and government securities by contractual maturity, as of December 31, 2020 were as follows (in thousands):

	Available-for-Sale
	Cost		Fair Value
Due in 1-year or less	$	137,588	$	137,567
Due after 1-year through 5-years		49,851		49,825
	$	187,439	$	187,392

As of December 31, 2019, we did not hold any investments with a contractual maturity over one year.

Property and Equipment, Net

12 Months Ended

Dec. 31, 2020

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5.	Property and Equipment, Net

Property and equipment, net consisted of the following:

	December 31,
	2020			2019
Laboratory equipment	$	15,985		$	15,140
Computer equipment		2,874			2,874
Furniture and office equipment		1,033			1,033
Leasehold improvements		27,977			27,977
Construction in progress		348			213
		48,217			47,237
Less: Accumulated depreciation and amortization		(34,320	)		(27,742	)
	$	13,897		$	19,495

Depreciation and amortization expense was $6,578, $7,603 and $7,862 for the years ended December 31, 2020, 2019 and 2018, respectively.

Accrued Expenses and Other Current Liabilities

12 Months Ended

Dec. 31, 2020

Payables And Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

6.	Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:

	December 31,
	2020		2019
Development and clinical manufacturing costs	$	6,339	$	5,605
Payroll and payroll-related costs		6,734		4,609
Facility and other		1,153		670
	$	14,226	$	10,884

Leases

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Leases

The Company leases real estate, primarily laboratory, office and manufacturing space. The Company’s leases have remaining terms ranging from less than 1 year to 3 years. Certain leases include one or more options to renew, exercised at the Company’s sole discretion, with renewal terms that can extend the lease from one year to five years. The Company evaluated the renewal options in its leases to determine if it was reasonably certain that the renewal option would be exercised, and therefore should be included in the calculation of the operating lease assets and operating lease liabilities. Given the Company’s current business structure, uncertainty of future growth, and the associated impact to real estate, the Company concluded that it is not reasonably certain that any renewal options would be exercised. Therefore, the operating lease assets and operating lease liabilities only contemplate the initial lease terms. All the Company’s leases qualify as operating leases.

In July 2019, the Company entered into a sublease agreement with a related party to sublease a portion of its office and laboratory space in Cambridge, Massachusetts. The term of the sublease agreement commenced in July 2019 and ends on the last day of the 24th calendar month following commencement, with no option to extend. The annual rent for the subleased premises will be approximately $1,200 in the first year and $1,300 in the second year, which is greater than the annual rent owed by the Company to the landlord for the leased premises. The sublessee is obligated to pay all real estate taxes and costs related to the subleased premises, including cost of operations, maintenance, repair, replacement and property management.

As of December 31, 2020, future undiscounted cash inflows under the sublease are as follows:

Year Ending December 31,
2021	$	633
Total	$	633

The Company concluded that the sublease is an operating lease. Consistent with the Company’s policy election for lessor operating leases, each lease component and its associated non-lease components is accounted for as a single lease component.

The following table summarizes the presentation in the Company’s consolidated balance sheets of its operating leases:

	As of December 31, 2020		As of December 31, 2019
Assets:
Operating lease assets	$	9,041	$	11,356

Liabilities:
Operating lease liabilities	$	5,115	$	4,456
Operating lease liabilities, net of current portion		10,561		15,676
Total operating lease liabilities	$	15,676	$	20,132

The following table summarizes the effect of lease costs in the Company’s consolidated statement of operations and comprehensive loss:

	For the Year Ended			For the Year Ended
	December 31, 2020			December 31, 2019
Operating lease costs	$	4,163		$	4,532
Short-term lease costs		1,457			1,878
Variable lease costs		2,890			3,022
Sublease income		(1,813	)		(890	)
Total lease costs	$	6,697		$	8,542

During the year ended December 31, 2018, the Company recognized $4,377 of rental expense related to office, laboratory, and manufacturing space. During the years ended December 31, 2020 and December 31, 2019, the Company made cash payments of $6,302 and $6,514 for operating leases, respectively.

As of December 31, 2020, future payments of operating lease liabilities are as follows (in thousands):

	As of December 31, 2020
2021		6,461
2022		6,390
2023		5,158
2024		—
2025 and thereafter		—
Total future payments of operating lease liabilities	$	18,009
Less: imputed interest		(2,333	)
Present value of operating lease liabilities	$	15,676

As of December 31, 2020, the weighted average remaining lease term was 2.89 years and the weighted average incremental borrowing rate used to determine the operating lease liability was 11%. As of December 31, 2019, the weighted average remaining lease term was 3.88 years and the weighted average incremental borrowing rate used to determine the operating lease liability was 11%.

Restructuring

12 Months Ended

Dec. 31, 2020

Restructuring And Related Activities [Abstract]

Restructuring

8.	Restructuring

In February 2019, the Company implemented corporate changes to focus its resources on advancing its clinical-stage therapeutic candidates. As a result, the Company is concentrating on completing the SER-287 Phase 2b study in mild-to-moderate UC, expanding the SER-109 safety database to meet the FDA threshold of at least 300 patients, advancing the SER-401 Phase 1b study, to evaluate augmenting checkpoint inhibitor response in patients with metastatic melanoma and advancing SER-301 into clinical development. In connection with the prioritization of these therapeutic candidates, the Company made changes to its management team and reduced headcount by approximately 30 percent.

During the year ended December 31, 2019 the Company recorded charges of $1,492, related to severance and other termination benefits. No restructuring charges were recorded during the years ended December 31, 2020 and 2018. During the year ended December 31, 2019 the Company paid $1,299 related to the restructuring and paid out the remaining $193 in 2020.

During the year ended December 31, 2019, the Company paid $1,299 related to the restructuring. The outstanding restructuring liabilities are included in accrued expenses and other current liabilities on the consolidated balance sheets as of December 31, 2019. As of December 31, 2019, the components of the outstanding restructuring liabilities included in accrued expenses and other current liabilities were as follows:

	Employee Severance and Other Benefits
Restructuring expenses	$	1,492
Cash payments	$	(1,299	)
Liability included in accrued expenses and other current liabilities at December 31, 2019	$	193

During the year ended December 31, 2020, the Company made the remaining cash payments of $193. There were no outstanding restructuring liabilities included in accrued expenses and other current liabilities as of December 31, 2020.

Notes Payable

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Notes Payable

9.	Notes Payable

On October 29, 2019 (the “Closing Date”), the Company entered into a Loan and Security Agreement (the “Loan Agreement”) with Hercules Capital, Inc. (“Hercules”) pursuant to which a term loan in an aggregate principal amount of up to $50,000 (the “Term Loan Facility”) is available to the Company in three tranches, subject to certain terms and conditions. The first tranche of $25,000 was advanced to the Company on the Closing Date. The Company did not meet the milestone requirements for the second tranche under the Term Loan Facility, and as such, the additional amount up to $12,500 is not available for the Company to borrow. The third tranche, which allows the Company to borrow an additional $12,500, will be available upon Hercules’ approval on or prior to June 30, 2021.

Advances under the Term Loan Facility will bear interest at a rate equal to the greater of either (i) the Prime Rate (as reported in The Wall Street Journal) plus 4.40%, and (ii) 9.65%. The Company will make interest only payments through December 1, 2021. The interest only period may be extended to June 1, 2022 upon satisfaction of certain milestones. Following the interest only period, the Company will repay the principal balance and interest of the advances in equal monthly installments through November 1, 2023.

The Company may prepay advances under the Loan Agreement, in whole or in part, at any time subject to a prepayment charge (the “Prepayment Premium”) equal to: (a) 3.0 % of amounts so prepaid, if such prepayment occurs during the first year following the Closing Date; (b) 2.0% of the amount so prepaid, if such prepayment occurs during the second year following the Closing Date, and (c) 1.0% of the amount so prepaid, if such prepayment occurs after the second year following the Closing Date.

Upon prepayment or repayment of all or any of the term loans under the Term Loan Facility, the Company will pay (in addition to any Prepayment Premium) an end of term charge of 4.85% of the aggregate funded amount under the Term Loan Facility. With respect to the first tranche, an end of term charge of $1,213 will be payable upon any prepayment or repayment. To the extent that the Company is provided additional advances under the Term Loan Facility, the 4.85% end of term charge will be applied to any such additional amounts.

The Term Loan Facility is secured by substantially all of the Company’s assets, other than the Company’s intellectual property. The Company has agreed to not pledge or secure its intellectual property to others.

Upon issuance, the first tranche was recorded as a liability with an initial carrying value of $24,575, net of debt issuance costs. The initial carrying value will be accreted to the repayment amount, which includes the outstanding principal plus the end of term charge, through interest expense using the effective interest rate method over the term of the debt. The effective interest rate is 11.47%. As of December 31, 2020, the carrying value of the debt is $25,093, of which $454 is classified as a current liability and $24,639 is classified as a non-current liability on the Company’s consolidated balance sheet.

As of December 31, 2020, the future principal payments due under the arrangement, excluding interest and the end of term charge, are as follows:

Year Ending December 31,	Principal
2021		949
2022		11,970
2023		12,081
Total	$	25,000

During the years ended December 31, 2020 and December 31, 2019, the Company recognized $2,899 and $502 of interest expense related to the Loan Agreement, respectively, which is reflected in other income (expense), net on the consolidated statements of operations and comprehensive loss.

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Convertible Preferred Stock

10.	Convertible Preferred Stock

On July 1, 2015, in connection with the closing of the initial public offering of the Company’s common stock (“IPO”), the Company effected its Restated Certificate of Incorporation, which authorizes the Company to issue 10,000,000 shares of preferred stock, $0.001 par value per share.

Stockholders' Equity Common Stock

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Stockholders' Equity Common Stock

11.	Stockholders’ Equity Common Stock

On July 1, 2015, in connection with the closing of the IPO, the Company effected its Restated Certificate of Incorporation, which authorizes the Company to issue 200,000,000 shares of common stock, $0.001 par value per share.

On June 18, 2019, the Company completed an underwritten public offering, in which the Company sold 26,666,667 shares of its common stock at a price to the public of $2.25 per share. The aggregate net proceeds received by the Company from the offering were approximately $55,976, after deducting underwriting discounts and commissions and offering expenses payable by the Company. In addition, the Company granted the underwriters a 30-day option to purchase up to an additional 2,666,666 shares of common stock at the public offering price, less underwriting discounts and commissions.

On June 21, 2019, the Company sold an additional 2,151,911 shares of its common stock at a price to the public of $2.25 per share. The aggregate net proceeds received by the Company were approximately $4,551, after deducting underwriting discounts and commissions and offering expenses payable by the Company.

On November 27, 2019, the Company entered into a Sales Agreement (the “2019 Sales Agreement”) with Cowen and Company, LLC (“Cowen”) to sell shares of the Company’s common stock, par value $0.001 per share (the “Common Stock”), with aggregate gross sales proceeds of up to $25,000, from time to time, through an “at the market” equity offering program under which Cowen will act as sales agent. On March 18, 2020, in connection with filing an updated registration statement on Form S-3 (File No. 333-237033), the Company entered into a Sales Agreement (the “2020 Sales Agreement”), with Cowen on substantially the same terms as the 2019 Sales Agreement and terminated the 2019 Sales Agreement. From January 1, 2020 to December 31, 2020 the Company sold 5,787,681 shares of common stock under the 2019 Sales Agreement and the 2020 Sales Agreement, as applicable, at an average price of approximately $4.40 per share, raising aggregate net proceeds of approximately $24,773 after deducting an aggregate commission of approximately 3%.

On August 12, 2020, the Company entered into an underwriting agreement (the “Underwriting Agreement”) with Cowen and Company, LLC and Piper Sandler & Co., as representatives of the several underwriters named therein (collectively, the “Underwriters”), in connection with the issuance and sale by the Company in a public offering of 10,500,000 shares of the Company’s common stock at a public offering price of $21.50 per share, less underwriting discounts and commissions, pursuant to an effective shelf registration statement on Form S-3 (Registration No. 333-244401) and a related prospectus supplement filed with the Securities and Exchange Commission (the “SEC” and such public offering, the “offering”). Under the terms of the Underwriting Agreement, the Company granted the Underwriters an option exercisable for 30 days to purchase up to an additional 1,575,000 shares of its common stock at the public offering price, less underwriting discounts and commissions, which the underwriters exercised in full. The Company received aggregate net proceeds from the offering of approximately $243,748 after deducting underwriting discounts and commissions and offering expenses payable by the Company.

Additionally on August 12, 2020, the Company entered into the Securities Purchase Agreement with Nestlé for the sale by the Company of 959,002 shares of the Company’s common stock at a purchase price of $20.855 per share (the “concurrent placement”). The Company received aggregate net proceeds from the concurrent placement of approximately $19,900 after deducting offering expenses payable by the Company. The consummation of the concurrent placement was contingent upon the closing of the offering and the satisfaction of certain other customary conditions. The shares were offered and sold to Nestlé pursuant to an effective registration statement on Form S-3 (File No. 333-237033) and a related prospectus supplement filed with the SEC.

2012 Stock Incentive Plan

The Company’s 2012 Stock Incentive Plan, as amended, (the “2012 Plan”) provided for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. The 2012 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. The Company generally granted stock-based awards with service conditions only (“service-based” awards).

Stock options granted under the 2012 Plan generally vest over four years and expire after ten years, although options have been granted with vesting terms less than four years. As of December 31, 2020, there were no shares available for future grant under the 2012 Plan.

2015 Incentive Award Plan

On June 16, 2015, the Company’s stockholders approved the 2015 Incentive Award Plan (the “2015 Plan”), which became effective on June 25, 2015. The 2015 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2015 Plan was the sum of (i) 2,200,000 shares of common stock and (ii) the number of shares subject to awards outstanding under the 2012 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company on or after the effective date of the 2015 Plan. In addition, the number of shares of common stock that may be issued under the 2015 Plan is subject to increase on the first day of each calendar year, beginning in 2016 and ending in 2025, equal to the lesser of (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the preceding applicable calendar year and (ii) an amount determined by the Company’s board of directors.

Stock options granted under the 2015 Plan generally vest over four years and expire after ten years, although options have been granted with vesting terms less than four years. As of December 31, 2020, there were 1,018,232 shares available for future grant under the 2015 Plan.

2015 Employee Stock Purchase Plan

On June 16, 2015, the Company’s stockholders approved the 2015 Employee Stock Purchase Plan (the “ESPP”), which became effective on June 25, 2015. A total of 365,000 shares of common stock were reserved for issuance under the ESPP. In addition, the number of shares of common stock that may be issued under the ESPP automatically increase on the first day of each calendar year, beginning in 2016 and ending in 2025, by an amount equal to the lesser of (i) 400,000 shares, (ii) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (iii) an amount determined by the Company’s board of directors. Offering periods under the ESPP will commence when determined by the plan administrator. As of December 31, 2020, there were 155,293 shares issued under the ESPP and 2,092,181 shares were reserved and available for issuance under the ESPP.

The ESPP provides that eligible employees may contribute up to 15% of their eligible earnings toward the semi-annual purchase of the Company's common stock. The ESPP is qualified under Section 423 of the Internal Revenue Code. The employee's purchase price is derived from a formula based on the closing price of the common stock on the first day of the offering period versus the closing price on the date of purchase (or, if not a trading day, on the immediately preceding trading day). The offering period under the ESPP has a duration of six months, and the purchase price with respect to each offering period beginning on or after such date is, until otherwise amended, equal to 85% of the lesser of (i) the fair market value of the Company's common stock at the commencement of the applicable six-month offering period or (ii) the fair market value of the Company's common stock on the purchase date. The Company estimates the fair value of common stock under the ESPP using a Black-Scholes valuation model. The fair value was estimated on the date of grant using the Black-Scholes option valuation model and the straight-line attribution approach with the following weighted-average assumptions: risk-free interest rate (1.2%); expected term (0.5 years); expected volatility (74.9%); and an expected dividend yield (0%). The Company recorded $200, $109 and $145 of stock-based compensation expense under the ESPP for the twelve months ended December 31, 2020, 2019 and 2018, respectively.

Stock Option Valuation

The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis:

	Year Ended December 31,
	2020			2019			2018
Risk-free interest rate		1.26	%		2.64	%		2.39	%
Expected term (in years)		6.0			6.0			6.0
Expected volatility		73.3	%		88.4	%		76.0	%
Expected dividend yield		0	%		0	%		0	%

Stock Options

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2020:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2019		8,310,683		$	10.36		7.01	$	3,427
Granted		5,095,365			7.11
Exercised		(2,214,011	)		6.51
Forfeited		(1,154,907	)		10.50
Outstanding as of December 31, 2020		10,037,130		$	9.54		7.87	$	156,627
Options exercisable as of December 31, 2020		3,164,129		$	14.70		5.75	$	34,652

The weighted average grant-date fair value of stock options granted during the years ended December 31, 2020, 2019 and 2018 was $5.08, $4.13, and $6.53 per share, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2020, 2019, and 2018 was $37,255, $244, and $1,801, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock.

During the year ended December 31, 2019, the Company granted performance-based stock options to employees for the purchase of an aggregate of 1.1 million shares of common stock with a grant date fair value of $4.58 per share. These stock options are exercisable only upon achievement of specified performance targets. As of December 31, 2020, none of these options were exercisable because none of the specified performance targets had been achieved. Because achievement of the specified performance targets was not deemed probable as of December 31, 2020, the Company did not record any expense for these stock options from the dates of issuance through December 31, 2020.

Restricted Stock Units

The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2020:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2019		130,000		$	8.86
Granted		6,500		$	25.36
Forfeited		(5,000	)	$	9.12
Vested		(125,000	)	$	2.29
Unvested restricted stock units as of December 31, 2020		6,500		$	25.36

The aggregate intrinsic value of restricted stock units that vested during the years ended December 31, 2020, 2019 and 2018 was $532, $517, and $1,206, respectively.

Stock-based Compensation

The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its consolidated statements of operations and comprehensive loss:

	Year Ended December 31,
	2020		2019		2018
Research and development expenses	$	4,760	$	4,613	$	8,388
General and administrative expenses		4,064		3,731		8,253
	$	8,824	$	8,344	$	16,641

As of December 31, 2020, the Company had an aggregate of $26,221 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 3.49 years.

Collaboration Revenue

12 Months Ended

Dec. 31, 2020

Revenue From Contract With Customer [Abstract]

Collaboration Revenue

12.	Collaboration Revenue

Nestlé Collaboration Agreement

Summary of Agreement

In January 2016, the Company entered into the License Agreement for the development and commercialization of certain product candidates in development for the treatment and management of CDI and IBD, including UC and Crohn’s disease. The License Agreement supports the development of the Company’s portfolio of products for CDI and IBD in markets outside of the United States and Canada (the “Licensed Territory”). The Company has retained full commercial rights to its entire portfolio of product candidates with respect to the United States and Canada.

Under the License Agreement, the Company granted to Nestlé an exclusive, royalty-bearing license to develop and commercialize, in the Licensed Territory, certain products based on its microbiome technology that are being developed for the

treatment of CDI and IBD, including SER-109, SER-262, SER-287 and SER-301 (collectively, the “Nestlé Collaboration Products”). The License Agreement sets forth the Company’s and Nestlé’s respective obligations for development, commercialization, regulatory and manufacturing and supply activities for the Nestlé Collaboration Products with respect to the licensed fields and the Licensed Territory.

Under the License Agreement, Nestlé agreed to pay the Company an upfront cash payment of $120,000, which the Company received in February 2016. The Company is eligible to receive up to $285,000 in development milestone payments, $375,000 in regulatory payments and up to an aggregate of $1,125,000 for the achievement of certain commercial milestones related to the sales of Nestlé Collaboration Products. Nestlé also agreed to pay the Company tiered royalties, at percentages ranging from the high single digits to high teens, of net sales of Nestlé Collaboration Products in the Licensed Territory.

Under the License Agreement, the Company was entitled to receive a $20,000 milestone payment from Nestlé following initiation of a SER-287 Phase 2 study and a $20,000 milestone payment from Nestlé following the initiation of a SER-287 Phase 3 study. In November 2018, the Company entered into a letter agreement with Nestlé which modified the License Agreement to address the current clinical plans for SER-287. Through the letter agreement, the Company and Nestlé agreed that following initiation of the SER-287 Phase 2b study, the Company will receive $40,000 in milestone payments from Nestlé which represents the milestone due to the Company for the initiation of the SER-287 Phase 2 and Phase 3 studies. The SER-287 Phase 2b study was initiated and the $40,000 of milestone payments were received in December 2018. The letter agreement also provides scenarios under which Nestlé’s reimbursement to the Company for certain Phase 3 development costs would be reduced or delayed depending on the outcomes of the SER-287 Phase 2b study.

Accounting Analysis

The Company assessed the License Agreement in accordance with ASC 606 and concluded that Nestlé is a customer. The Company identified the following promises under the contract: (i) a license to develop and commercialize the Nestlé Collaboration Products in the Licensed Territory, (ii) obligation to perform research and development services, (iii) participation on a joint steering committee, and (iv) manufacturing services to provide clinical supply to complete future clinical trials. In addition, the Company identified a contingent obligation to perform manufacturing services to provide commercial supply if commercialization occurs, which is contingent upon regulatory approval. This contingent obligation is not a performance obligation at inception and has been excluded from the initial allocation as it represents a separate buying decision at market rates, rather than a material right in the contract. The Company assessed the promised goods and services to determine if they are distinct. Based on this assessment, the Company determined that Nestlé cannot benefit from the promised goods and services separately from the others as they are highly interrelated and therefore not distinct. Accordingly, the promised goods and services represent one combined performance obligation and the entire transaction price will be allocated to that single combined performance obligation.

At contract inception, the Company determined that the $120,000 non-refundable upfront amount constituted the entirety of the consideration to be included in the transaction price as the development, regulatory, and commercial milestones were fully constrained. During the year ended December 31, 2016, the Company received $10,000 from Nestlé in connection with the initiation of the Phase 1b study for SER-262 in CDI. During the year ended December 31, 2017, the Company received $20,000 from Nestlé in connection with the initiation of the Phase 3 study for SER-109. During the year ended December 31, 2018, the Company received $40,000 from Nestlé in connection with the initiation of the Phase 2b study for SER-287. During the year ended December 31, 2020, the Company received $10,000 from Nestlé in connection with the initiation of the Phase 1b SER-301 study.

Therefore, as of December 31, 2020, the aggregate amount of the transaction price allocated to the remaining performance obligation of the License Agreement was approximately $200,000.

During the year ended December 31, 2018, the Company received $40,000 from Nestlé in connection with the initiation of the Phase 2b study for SER-287. In the third quarter of 2018, the Company increased the transaction price by $20,000 associated with the Phase 2 milestone for SER-287. The Company estimated the $20,000 of variable consideration by using the most likely amount method which best predicts the amount of consideration to which the Company will be entitled. The Company included the $20,000 in the transaction price because it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur when the uncertainty associated with the variable consideration was subsequently resolved.

In the fourth quarter of 2018, the Company entered into a letter agreement with Nestlé which modified the License Agreement to address the current clinical plans for SER-287. As a result of this modification, the Company and Nestlé agreed that the $20,000

milestone payment due to the Company from Nestlé following the initiation of a Phase 3 study for SER-287 would now be due to the Company upon initiation of the SER-287 Phase 2b study. The Letter Agreement constituted a contract modification under ASC 606. The Company accounted for the contract modification through a cumulative catch-up adjustment of approximately $5,517 because the contract modification did not add any additional goods or services and the remaining goods and services are not distinct. The SER-287 Phase 2b study was initiated in December 2018 and the Company included the $20,000 in the transaction price as of December 31, 2018. The transaction price as of December 31, 2018 was approximately $190,000.

In April 2019, the Company, with the approval of the Seres/ Nestlé Joint Steering Committee, as provided for in the License Agreement, modified the SER-109 clinical trial. As a result of this modification, the Company and Nestlé agreed, and informed the FDA, that the target study enrollment would be reduced from 320 subjects to 188 subjects. This modification to the SER-109 clinical trial constituted a contract modification under ASC 606. The Company accounted for the contract modification through a cumulative catch-up adjustment because the contract modification did not add any additional goods or services and the remaining goods and services are not distinct. The modification reduced the total estimated costs in the Company’s cost-to-cost model for the License Agreement and resulted in the Company recognizing $6,830 of collaboration revenue – related party in the twelve months ended December 31, 2019.

During the year ended December 31, 2020, the Company received $10,000 from Nestlé in connection with the initiation of the Phase 1b study for SER-301. In the third quarter of 2020, the Company increased the transaction price by $10,000 associated with the Phase 1b milestone for SER-301. The Company estimated the $10,000 of variable consideration by using the most likely amount method which best predicts the amount of consideration to which the Company will be entitled. The Company included the $10,000 in the transaction price because it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur when the uncertainty associated with the variable consideration was subsequently resolved. This resulted in $4,565 of cumulative catch-up revenue during the twelve months ended December 31, 2020, which was primarily off-set by the Company’s updated estimate of future costs that would be incurred from on-going research and development services to complete its performance obligation under the License Agreement that is recognized over time using the input method.

During the twelve months ended December 31, 2020, 2019, and 2018 using the cost-to-cost method, which best depicts the transfer of control to the customer, the Company recognized $11,897, $27,188, and $26,917 of Collaboration revenue – related party, respectively.

As of December 31, 2020 and December 31, 2019, there was $108,174, and $110,071 of deferred revenue related to the unsatisfied portion of the performance obligation under the License Agreement. As of December 31, 2020, deferred revenue is classified as current or non-current in the consolidated balance sheets based on the Company’s estimate of revenue that will be recognized within the next twelve months which is determined by the cost-to-cost method which measures the extent of progress towards completion based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the performance obligation. Due to the nature of the work required to be performed to satisfy the performance obligation, the Company’s estimation of costs expected is complex and requires significant judgment. All costs associated with the License Agreement are recorded in research and development expense in the consolidated statements of operations and comprehensive loss.

AstraZeneca Research Collaboration and Option Agreement

Summary of the Agreement

In March 2019, the Company entered into a Research Collaboration and Option Agreement (the “Research Agreement”) with MedImmune, LLC, a wholly owned subsidiary of AstraZeneca Inc. (“AstraZeneca”), to advance the mechanistic understanding of the microbiome in augmenting the efficacy of cancer immunotherapy. Under the Research Agreement, the Company and AstraZeneca will conduct certain research and development activities as set forth on a research plan focused on the role of the microbiome in certain cancers and cancer immunotherapies, including furthering the research program for SER-401, in combination with AstraZeneca compounds targeting various cancers.

Pursuant to the Research Agreement, the Company agreed not to conduct research or development on any microbiome products specifically designed by the Company during the term of the Research Agreement for the treatment of cancer (“Microbiome Oncology Products”), with or on behalf of any third-party without the prior approval of the joint steering committee for the Research Agreement for at least three years after the effective date (the “Exclusivity Period”). Additionally, AstraZeneca will pay to the Company a total of $20,000 in three equal installments, the first of which the Company received in April 2019, the second of which the Company received in December 2019, and the third of which the Company received in January 2021. Such payments are payable even if the

Research Agreement is terminated in accordance with its terms, unless the Research Agreement is terminated by AstraZeneca for the Company’s uncured material breach. Additionally, AstraZeneca will bear its costs of conducting activities under the research plan and will reimburse the Company for all activities performed under the research plan based on actual full-time employee (“FTE”) time and certain third-party costs incurred by the Company in connection therewith.

Under the Research Agreement, the Company granted to AstraZeneca an exclusive option to negotiate a worldwide, sublicensable exclusive license under relevant intellectual property rights controlled by the Company to exploit Microbiome Oncology Products for the treatment of cancer. Additionally, the Company granted to AstraZeneca an additional exclusive option to obtain a worldwide, sublicensable, license under certain intellectual property rights arising out of the Agreement or coming into the control of the Company during the term of the Agreement, to exploit AstraZeneca’s oncology and other assets which are the subject of the research plan. AstraZeneca may exercise each option at any point prior to 90 days after the end of the Exclusivity Period (the “Option Exercise Period”) by delivering an option exercise notice to the Company. If AstraZeneca exercises an option during the Option Exercise Period, the parties will enter into exclusive, good faith negotiations for a period of six months (the “Negotiation Period”) regarding the terms of the definitive license agreement contemplated by such option. If no definitive agreement is reached during the Negotiation Period, subject to certain other terms and conditions applicable for a one (1) year period, the Company is free to license, further develop or otherwise exploit its assets that were the subject of the option without further obligation to AstraZeneca.

The term of the Research Agreement continues in effect until the Research Agreement is terminated by the parties in accordance with its terms by mutual written agreement. Either party may terminate the Research Agreement for the other party’s uncured material breach or bankruptcy or insolvency-related events. AstraZeneca may terminate the Research Agreement for convenience. In December 2020, the Company received written notice from AstraZeneca that AstraZeneca elected to terminate the Research Agreement by and in accordance with its terms. The termination of the Research Agreement will be effective on April 2, 2021 (the “Termination Date”), which is 120 days from the date of the termination notice.

Accounting Analysis

The Company assessed the Research Agreement in accordance with ASC 606 and concluded that AstraZeneca is a customer. The Company identified the following promises under the contract: (i) a research license, (ii) an obligation to perform research and development services, and (iii) participation on a joint steering committee. The Company assessed the promised goods and services to determine if they are distinct. Based on this assessment, the Company determined that AstraZeneca cannot benefit from the promised goods and services separately from the others as they are highly interrelated and therefore not distinct. Accordingly, the promised goods and services represent one combined performance obligation and the entire transaction price will be allocated to that single combined performance obligation.

Each exclusive option granted to AstraZeneca provides AstraZeneca with the right to negotiate a license agreement in the future at fair value. Therefore, the Company concluded that each option does not constitute a performance obligation at inception and has been excluded from the initial allocation since each option represents a separate buying decision at market rates, rather than a material right in the contract.

At contract inception, the Company determined that the transaction price is comprised of: (i) the $20,000 fee, which represents fixed consideration, and (ii) the estimated reimbursement of research and development costs incurred, which represents variable consideration. The Company included the estimated reimbursement of research and development costs, approximately $13,900, in the transaction price at the inception of the arrangement because the Company is required to perform research and development services and the contract requires AstraZeneca to reimburse the Company for costs incurred. Also, since the related revenue would be recognized only as the costs are incurred, and the contract precludes the joint steering committee from changing the research plan without mutual agreement, the Company determined it is not probable that a significant reversal of cumulative revenue would occur.

The Company determined that revenue under the Research Agreement should be recognized over time as AstraZeneca simultaneously receives the benefit from the Company as the Company performs under the single performance obligation over time. The Company will recognize revenue for the single performance obligation using a cost-to-cost input method as the Company has concluded it best depicts the research and joint steering committee participation services performed prior to AstraZeneca’s ability to negotiate a license. Under this method, the transaction price is recognized over the contract’s entire performance period, using costs incurred relative to total estimated costs to determine the extent of progress towards completion.

In December 2020, the Company received written notice that AstraZeneca elected to terminate the Research Agreement. As a result of AstraZeneca’s decision to terminate the Research Agreement, the Company’s performance obligations under the Research Agreement have ended as of December 31, 2020. The final transaction price of $23,377 is comprised of the $20,000 fixed consideration and $3,376 for the reimbursed research and development costs. The Company removed all costs associated with its remaining performance from the cost-to-cost model in the fourth quarter of 2020. This resulted in the Company recognizing the remaining deferred revenue of $15,145 to collaboration revenue in the year ended December 31, 2020. For the twelve months ended December 31, 2020, the Company recognized collaboration revenue of $17,161.

All costs associated with the Research Agreement are recorded in research and development expense in the consolidated statements of operations and comprehensive loss.

Contract Balances from Contracts with Customers

The following tables present changes in the Company’s contract liabilities during the twelve months ended December 31, 2020 and 2019:

	Balance as of December 31, 2019		Additions		Deductions			Balance as of December 31, 2020
Year ended December 31, 2020
Contract liabilities:
Deferred revenue - related party	$	110,071		10,000		(11,897	)	$	108,174
Deferred revenue	$	9,668		7,493		(17,161	)	$	-

	Balance as of December 31, 2018		Additions		Deductions			Balance as of December 31, 2019
Year ended December 31, 2019
Contract liabilities:
Deferred revenue - related party	$	137,259		—		(27,188	)	$	110,071
Deferred revenue	$	-		15,883		(6,215	)	$	9,668

During the twelve months ended December 31, 2020 the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods:

	Year Ended December 31,
	2020		2019
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$	21,565	$	27,188

When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded. Revenue is recognized from the contract liability over time using the cost-to-cost method.

- Definition

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-URI http://asc.fasb.org/topic&trid=49130388

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130551-203045

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130556-203045

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130554-203045

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130550-203045

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 12
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130554-203045

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+ Details

Net Loss per Share

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Net Loss per Share

13.	Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows:

	Year Ended December 31,
	2020			2019			2018
Numerator:
Net loss attributable to common stockholders	$	(89,127	)	$	(70,279	)	$	(98,942	)
Denominator:
Weighted average common shares outstanding, basic and diluted		79,789,220			56,649,220			40,743,492
Net loss per share attributable to common stockholders, basic and diluted	$	(1.12	)	$	(1.24	)	$	(2.43	)

The Company’s potential dilutive securities, which include stock options, unvested restricted common stock and shares issuable under the ESPP, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The following potential common shares, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:

	Year Ended December 31,
	2020		2019		2018
Stock options to purchase common stock		10,037,130		8,310,683		7,561,719
Unvested restricted stock units		6,500		130,000		226,900
Shares issuable under employee stock purchase plan		10,786		89,821		49,495
		10,054,416		8,530,504		7,838,114

Commitments and Contingencies

12 Months Ended

Dec. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

14.	Commitments and Contingencies

Leases

Refer to Note 7 “Leases” for discussion of the commitments associated with the Company’s lease portfolio.

Indemnification Agreements

In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third-parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2020 or 2019.

Legal Contingencies

The Company accrues a liability for legal contingencies when it believes that it is both probable that a liability has been incurred and that the Company can reasonably estimate the amount of the loss. The Company reviews these accruals and adjusts them to reflect ongoing negotiations, settlements, rulings, advice of legal counsel and other relevant information. To the extent new information is obtained and the views on the probable outcomes of claims, suits, assessments, investigations or legal proceedings change, changes in the Company’s accrued liabilities would be recorded in the period in which such determination is made.

In addition, in accordance with the relevant authoritative guidance, for any matters in which the likelihood of material loss is at least reasonably possible, the Company will provide disclosure of the possible loss or range of loss. If a reasonable estimate cannot be made, however, the Company will provide disclosure to that effect. The Company expenses legal costs as they are incurred.

The Company did not accrue any liabilities related to legal contingencies in its consolidated financial statements as of December 31, 2020 and December 31, 2019.

Income Taxes

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income Taxes

15.	Income Taxes

During the years ended December 31, 2020, 2019 and 2018, the Company recorded no income tax benefits for the net operating losses incurred in each year or interim period, due to its uncertainty of realizing a benefit from those items.

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:

	Year Ended December 31,
	2020			2019			2018
Federal statutory income tax rate		(21.0	)%		(21.0	)%		(21.0	)%
Research and development tax credits		(6.4	)		(5.8	)		(7.2	)
State taxes, net of federal benefit		(7.8	)		(6.8	)		(7.2	)
Stock-based compensation		(5.8	)		(1.7	)		2.0
Other		0.2			(0.7	)		0.8
Change in deferred tax asset valuation allowance		40.8			36.0			32.6
Effective income tax rate		—	%		—	%		—	%

Net deferred tax assets as of December 31, 2020 and 2019 consisted of the following:

	December 31,
	2020			2019
Deferred tax assets:
Net operating loss carryforwards	$	106,351		$	72,752
Research and development tax credit carryforwards		42,311			36,602
Capitalized organization costs		214			244
Stock-based compensation expense		11,615			12,783
Lease Liability		4,283			5,500
Charitable Contributions		15			13
Deferred Revenue		29,553			32,713
Accrued expenses		1,783			1,251
Capitalized research and development expenses		51			58
Section 163(j) limitation		540			-
Total deferred tax assets	$	196,716		$	161,916
Deferred tax liabilities:
Depreciation and amortization		(510	)		(1,467	)
Right of use assets		(2,470	)		(3,103	)
Total deferred tax liabilities		(2,980	)		(4,570	)
Valuation allowance	$	(193,736	)	$	(157,346	)
Net deferred tax assets	$	—		$	—

As of December 31, 2020, the Company had net operating loss carryforwards (“NOLs”) for federal and state income tax purposes of $390,000 and $386,900, respectively. Federal NOLs of $119,781, generated before 2018, will begin expiring in varying amounts in 2035 unless utilized. The remaining federal NOLs of $270,219, generated after 2017, will be carried forward indefinitely and could be used to offset up to 100% of taxable income of each future tax year for tax years before January 1, 2021 and up to 80% of taxable income in all other future tax years. Massachusetts does not follow federal time periods for NOLs and as such the Company’s Massachusetts NOLs of $386,900 will expire at various times starting in 2035. As of December 31, 2020, the Company

also had available research and development tax credit carryforwards for federal and state income tax purposes of $36,400 and $7,500, respectively, which begin to expire in 2031 and 2028, respectively. The federal research and development tax credits include an orphan drug credit carryforward of $20,700. Utilization of the NOLs and research and development tax credit carryforwards may be subject to a substantial annual limitation under Sections 382 and 383 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50% over a three-year period. Since its formation, the Company has raised capital through the issuance of capital stock on several occasions. These financings, combined with the purchasing shareholders' subsequent disposition of those shares, may have resulted in a change of control or could result in a change of control in the future upon subsequent disposition. The Company conducted an analysis to determine if historical changes in ownership through August 31, 2015 would limit or otherwise restrict its ability to utilize these NOLs and research and development credit carryforwards. As a result of this analysis, the Company does not believe there are any significant limitations on its ability to utilize these carryforwards. However, future changes in ownership after August 31, 2015 could affect the limitation in future years. Any limitation may result in expiration of a portion of the NOLs or research and development credit carryforwards before utilization.

The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2020 and 2019. Management reevaluates the positive and negative evidence at each reporting period.

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2020, 2019 and 2018 related primarily to the increases in NOLs, research and development tax credit carryforwards, stock-based compensation and decrease of the deferred rate due to tax reform were as follows:

	Year Ended December 31,
	2020			2019			2018
Valuation allowance at beginning of year	$	(157,346	)	$	(132,009	)	$	(94,126	)
Decreases recorded as benefit to income tax provision		—			—			—
Increases recorded to income tax provision		(36,390	)		(25,337	)		(37,883	)
Valuation allowance as of end of year	$	(193,736	)	$	(157,346	)	$	(132,009	)

The Company had no unrecognized tax benefits or related interest and penalties accrued for the years ended December 31, 2020 and 2019. The Company will recognize interest and penalties related to uncertain tax positions in income tax expense.

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. The Company is currently under examination by the Internal Revenue Service ("IRS") for the period ended December 31, 2018 related to its R&D tax credits. The Company's tax years are still open under statute from 2011 to present. All years may be examined to the extent the tax credit or net operating loss carryforwards are used in future periods.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Related Party Transactions

12 Months Ended

Dec. 31, 2020

Related Party Transactions

16.	Related Party Transactions

As described in Note 12, in January 2016 the Company entered into the License Agreement and, in November 2018, a letter agreement with Nestlé, successor in interest to Nestec, Ltd., for the development and commercialization of certain product candidates in development for the treatment and management of CDI and IBD, including UC and Crohn’s disease. Nestlé is a related party since Nestlé is one of the Company’s significant stockholders and is related to Nestlé Health Science U.S. Holdings, Inc, another significant stockholder. During the years ended December 31, 2020, 2019, and 2018, the Company recognized $11,897, $27,188, and $26,917, respectively, of related party revenue associated with the License Agreement. As of December 31, 2020 and 2019, there was $108,174 and $110,071, respectively, of deferred revenue related to the License Agreement, which is classified as current or non-current in the consolidated balance sheets. The Company has made no payments to Nestlé during the year ended December 31, 2020. There is no amount due from Nestlé as of December 31, 2020.

In July 2019, the Company entered into a sublease agreement with Flagship Pioneering, one of the Company’s significant stockholders, to sublease a portion of its office and laboratory space in Cambridge, Massachusetts. The term of the sublease agreement commenced in July 2019 and ends on the last day of the 24th calendar month following commencement, with no option to extend (see Note 7). Under this agreement, the Company recorded other income of $1,813 and $890 during the twelve months ended December 31, 2020 and 2019. The Company received cash payments of $1,813 and $890 during the twelve months ended December 31, 2020 and 2019.

401(k) Savings Plan

12 Months Ended

Dec. 31, 2020

Compensation And Retirement Disclosure [Abstract]

401(k) Savings Plan

17.	401(k) Savings Plan

The Company has a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Effective January 1, 2016, the Company elected to match 50% of the first 6% of an employee’s deferral. Company contributions are expensed in the year for which they are declared. During the years ended December 31, 2020, 2019, and 2018 the Company recorded expense of $586, $542, and $604, respectively, for 401(k) match contributions.

- Definition

The entire disclosure for retirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 55
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Use of Estimates

Cash Equivalents

Investments

Restricted Investments

Concentration of Credit Risk and of Significant Suppliers

Fair Value Measurements

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Property and Equipment

Impairment of Long-Lived Assets

Research and Development Costs

Research Contract Costs and Accruals

Patent Costs

Accounting for Stock-Based Compensation

The Company accounts for forfeitures of stock-based awards as they occur rather than applying an estimated forfeiture rate to stock-based compensation expense.

Revenue Recognition

The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption.

(i)

identify the contract(s) with a customer;

(ii)

identify the performance obligations in the contract;

(iii)

determine the transaction price;

(iv)

allocate the transaction price to the performance obligations in the contract; and

(v)

recognize revenue when (or as) the entity satisfies a performance obligation.

Collaborative Revenue

Collaboration Revenue

Licenses of intellectual property

Milestone Payments

Royalties

Manufacturing supply services

Recently Adopted and Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

•

This standard became effective for the Company on January 1, 2020 and did not have a material impact on the Company’s consolidated financial statements and related disclosures.

Recently Issued Accounting Pronouncements

Fair Value of Financial Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Summary of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis

The following tables present the Company’s fair value hierarchy for its assets and liabilities that are measured at fair value on a recurring basis (in thousands):

	Fair Value Measurements as of December 31, 2020 Using:
	Level 1		Level 2		Level 3		Total
Cash Equivalents:
Money market funds	$	35,480	$	—	$	—	$	35,480
Commercial paper		—		10,313		—		10,313
Corporate bonds		—		2,014		—		2,014
Investments:
Commercial paper	$	—	$	12,343	$	—	$	12,343
Corporate bonds		—		68,289		—		68,289
Certificate of deposits		—		2,272		—		2,272
Government securities		—		104,488		—		104,488
	$	35,480	$	199,719	$	—	$	235,199

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Cash Equivalents:
Money market funds	$	25,510	$	—	$	—	$	25,510
Commercial paper		—		4,243		—		4,243
Corporate bonds		—		4,900		—		4,900
Investments:
Commercial paper	$	—	$	11,957			$	11,957
Corporate bonds		—		17,733				17,733
	$	25,510	$	38,833	$	—	$	64,343

Investments (Tables)

12 Months Ended

Dec. 31, 2020

Investments Debt And Equity Securities [Abstract]

Schedule of Investments by Security Type

Investments by security type consisted of the following at December 31, 2020 and December 31, 2019 (in thousands):

	December 31, 2020
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Commercial paper	$	12,343	$	—	$	—		$	12,343
Corporate bonds		68,333		8		(52	)		68,289
Certificate of deposits		2,272		-		-			2,272
Government securities		104,491		6		(9	)		104,488
	$	187,439	$	14	$	(61	)	$	187,392

	December 31, 2019
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Commercial paper	$	11,957	$	—	$	—		$	11,957
Corporate bonds		17,732		3		(2	)		17,733
	$	29,689	$	3	$	(2	)	$	29,690

Schedule of Amortized Cost and Fair Value of Investments by Contractual Maturity

	Available-for-Sale
	Cost		Fair Value
Due in 1-year or less	$	137,588	$	137,567
Due after 1-year through 5-years		49,851		49,825
	$	187,439	$	187,392

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2020

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following:

	December 31,
	2020			2019
Laboratory equipment	$	15,985		$	15,140
Computer equipment		2,874			2,874
Furniture and office equipment		1,033			1,033
Leasehold improvements		27,977			27,977
Construction in progress		348			213
		48,217			47,237
Less: Accumulated depreciation and amortization		(34,320	)		(27,742	)
	$	13,897		$	19,495

Accrued Expenses and Other Current Liabilities (Tables)

12 Months Ended

Dec. 31, 2020

Payables And Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:

	December 31,
	2020		2019
Development and clinical manufacturing costs	$	6,339	$	5,605
Payroll and payroll-related costs		6,734		4,609
Facility and other		1,153		670
	$	14,226	$	10,884

Leases (Tables)

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Summary of Future Undiscounted Cash Inflows Under the Sublease

As of December 31, 2020, future undiscounted cash inflows under the sublease are as follows:

Year Ending December 31,
2021	$	633
Total	$	633

Summary of Operating Lease Assets and Liabilities

The following table summarizes the presentation in the Company’s consolidated balance sheets of its operating leases:

	As of December 31, 2020		As of December 31, 2019
Assets:
Operating lease assets	$	9,041	$	11,356

Liabilities:
Operating lease liabilities	$	5,115	$	4,456
Operating lease liabilities, net of current portion		10,561		15,676
Total operating lease liabilities	$	15,676	$	20,132

Summary of Lease Costs

The following table summarizes the effect of lease costs in the Company’s consolidated statement of operations and comprehensive loss:

	For the Year Ended			For the Year Ended
	December 31, 2020			December 31, 2019
Operating lease costs	$	4,163		$	4,532
Short-term lease costs		1,457			1,878
Variable lease costs		2,890			3,022
Sublease income		(1,813	)		(890	)
Total lease costs	$	6,697		$	8,542

Schedule of Future Payments of Operating Lease Liabilities

As of December 31, 2020, future payments of operating lease liabilities are as follows (in thousands):

	As of December 31, 2020
2021		6,461
2022		6,390
2023		5,158
2024		—
2025 and thereafter		—
Total future payments of operating lease liabilities	$	18,009
Less: imputed interest		(2,333	)
Present value of operating lease liabilities	$	15,676

Restructuring (Tables)

12 Months Ended

Dec. 31, 2020

Restructuring And Related Activities [Abstract]

Schedule of Components of Outstanding Restructuring Liabilities

As of December 31, 2019, the components of the outstanding restructuring liabilities included in accrued expenses and other current liabilities were as follows:

	Employee Severance and Other Benefits
Restructuring expenses	$	1,492
Cash payments	$	(1,299	)
Liability included in accrued expenses and other current liabilities at December 31, 2019	$	193

Notes Payable (Tables)

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Summary of Future Principal Payments Due Under Arrangement, Excluding Interest and End of Term Charge

As of December 31, 2020, the future principal payments due under the arrangement, excluding interest and the end of term charge, are as follows:

Year Ending December 31,	Principal
2021		949
2022		11,970
2023		12,081
Total	$	25,000

Stockholders' Equity Common Stock (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Summary of Fair Value of Stock Option

The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis:

	Year Ended December 31,
	2020			2019			2018
Risk-free interest rate		1.26	%		2.64	%		2.39	%
Expected term (in years)		6.0			6.0			6.0
Expected volatility		73.3	%		88.4	%		76.0	%
Expected dividend yield		0	%		0	%		0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2020:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2019		8,310,683		$	10.36		7.01	$	3,427
Granted		5,095,365			7.11
Exercised		(2,214,011	)		6.51
Forfeited		(1,154,907	)		10.50
Outstanding as of December 31, 2020		10,037,130		$	9.54		7.87	$	156,627
Options exercisable as of December 31, 2020		3,164,129		$	14.70		5.75	$	34,652

Summary of Restricted Stock Activity

The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2020:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2019		130,000		$	8.86
Granted		6,500		$	25.36
Forfeited		(5,000	)	$	9.12
Vested		(125,000	)	$	2.29
Unvested restricted stock units as of December 31, 2020		6,500		$	25.36

Summary of Stock Based Compensation Expense

	Year Ended December 31,
	2020		2019		2018
Research and development expenses	$	4,760	$	4,613	$	8,388
General and administrative expenses		4,064		3,731		8,253
	$	8,824	$	8,344	$	16,641

Collaboration Revenue (Tables)

12 Months Ended

Dec. 31, 2020

Revenue From Contract With Customer [Abstract]

Changes in Contract Liabilities

The following tables present changes in the Company’s contract liabilities during the twelve months ended December 31, 2020 and 2019:

	Balance as of December 31, 2019		Additions		Deductions			Balance as of December 31, 2020
Year ended December 31, 2020
Contract liabilities:
Deferred revenue - related party	$	110,071		10,000		(11,897	)	$	108,174
Deferred revenue	$	9,668		7,493		(17,161	)	$	-

	Balance as of December 31, 2018		Additions		Deductions			Balance as of December 31, 2019
Year ended December 31, 2019
Contract liabilities:
Deferred revenue - related party	$	137,259		—		(27,188	)	$	110,071
Deferred revenue	$	-		15,883		(6,215	)	$	9,668

During the twelve months ended December 31, 2020 the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods:

	Year Ended December 31,
	2020		2019
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$	21,565	$	27,188

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Basic and Diluted Net Loss per Share Attributable to Common Stockholders

Basic and diluted net loss per share attributable to common stockholders was calculated as follows:

	Year Ended December 31,
	2020			2019			2018
Numerator:
Net loss attributable to common stockholders	$	(89,127	)	$	(70,279	)	$	(98,942	)
Denominator:
Weighted average common shares outstanding, basic and diluted		79,789,220			56,649,220			40,743,492
Net loss per share attributable to common stockholders, basic and diluted	$	(1.12	)	$	(1.24	)	$	(2.43	)

Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share

The following potential common shares, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:

	Year Ended December 31,
	2020		2019		2018
Stock options to purchase common stock		10,037,130		8,310,683		7,561,719
Unvested restricted stock units		6,500		130,000		226,900
Shares issuable under employee stock purchase plan		10,786		89,821		49,495
		10,054,416		8,530,504		7,838,114

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:

	Year Ended December 31,
	2020			2019			2018
Federal statutory income tax rate		(21.0	)%		(21.0	)%		(21.0	)%
Research and development tax credits		(6.4	)		(5.8	)		(7.2	)
State taxes, net of federal benefit		(7.8	)		(6.8	)		(7.2	)
Stock-based compensation		(5.8	)		(1.7	)		2.0
Other		0.2			(0.7	)		0.8
Change in deferred tax asset valuation allowance		40.8			36.0			32.6
Effective income tax rate		—	%		—	%		—	%

Schedule of Net Deferred Tax Assets

Net deferred tax assets as of December 31, 2020 and 2019 consisted of the following:

	December 31,
	2020			2019
Deferred tax assets:
Net operating loss carryforwards	$	106,351		$	72,752
Research and development tax credit carryforwards		42,311			36,602
Capitalized organization costs		214			244
Stock-based compensation expense		11,615			12,783
Lease Liability		4,283			5,500
Charitable Contributions		15			13
Deferred Revenue		29,553			32,713
Accrued expenses		1,783			1,251
Capitalized research and development expenses		51			58
Section 163(j) limitation		540			-
Total deferred tax assets	$	196,716		$	161,916
Deferred tax liabilities:
Depreciation and amortization		(510	)		(1,467	)
Right of use assets		(2,470	)		(3,103	)
Total deferred tax liabilities		(2,980	)		(4,570	)
Valuation allowance	$	(193,736	)	$	(157,346	)
Net deferred tax assets	$	—		$	—

Summary of Changes In Valuation Allowance for Deferred Tax Assets

	Year Ended December 31,
	2020			2019			2018
Valuation allowance at beginning of year	$	(157,346	)	$	(132,009	)	$	(94,126	)
Decreases recorded as benefit to income tax provision		—			—			—
Increases recorded to income tax provision		(36,390	)		(25,337	)		(37,883	)
Valuation allowance as of end of year	$	(193,736	)	$	(157,346	)	$	(132,009	)

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands					1 Months Ended	12 Months Ended
	Aug. 12, 2020 USD ($) $ / shares shares	Aug. 10, 2020 Patient	Jun. 21, 2019 USD ($) $ / shares shares	Jun. 18, 2019 USD ($) $ / shares shares	Feb. 28, 2019 Patient	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporated month and year						2010-10
Entity incorporation, state or country code						DE
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs						$ 243,748	$ 60,527
Accumulated deficit						548,776	459,649
Net loss						(89,127)	(70,279)	$ (98,942)
Cash used in operations						(93,610)	$ (76,520)	$ (62,854)
Cash, cash equivalents and short and long-term investments						$ 303,441
Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares \| shares						12,075,000	28,818,578
Underwritten Public Offering [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs	$ 243,748		$ 4,551	$ 55,976
Underwritten Public Offering [Member] \| Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares \| shares	10,500,000		2,151,911	26,666,667
Common stock, share price \| $ / shares	$ 21.50		$ 2.25	$ 2.25
Concurrent Placement [Member] \| Securities Purchase Agreement [Member] \| Nestlé [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares \| shares	959,002
Common stock, share price \| $ / shares	$ 20.855
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs	$ 19,900
Maximum [Member] \| Underwritten Public Offering [Member] \| Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Option to purchase additional shares of common stock \| shares	1,575,000			2,666,666
SER-109 [Member] \| Minimum [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Number of patients \| Patient		300			300

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Accounting Policies [Line Items]
Maximum maturity days for cash equivalents	90 days
Restricted investments	$ 1,400,000	$ 1,400,000
Impairment losses on long lived assets	0
Adoption of new revenue standard (ASC 606)	(174,750,000)	48,324,000	$ 48,045,000		$ (60,699,000)
Accounts receivable	1,186,000	406,000
Unbilled accounts receivable	8,201,000	$ 1,379,000
Revenue related to sales-based royalties	0
Revenue related to milestone payments	$ 0
Minimum percentage of likelihood of tax benefit being realized upon settlement	50.00%
As reported under Topic 842 [Member]
Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2019
Change in accounting principle, accounting standards update, transition option elected	us-gaap:AccountingStandardsUpdate201602RetrospectiveMember
ASU 2018-18 [Member]
Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect	true
ASU 2018-13 [Member]
Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect	true
Revision of Prior Period, Accounting Standards Update, Adjustment [Member] \| ASU 2014-09 [Member]
Accounting Policies [Line Items]
Adoption of new revenue standard (ASC 606)				$ 26,857,000
Laboratory Equipment [Member]
Accounting Policies [Line Items]
Useful life of asset	5 years
Computer Equipment [Member]
Accounting Policies [Line Items]
Useful life of asset	3 years
Furniture and Office Equipment [Member]
Accounting Policies [Line Items]
Useful life of asset	3 years

Fair Value of Financial Assets and Liabilities - Summary of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments:
Investments	$ 187,392	$ 29,690
Fair Value, Measurements, Recurring [Member]
Investments:
Investments	235,199	64,343
Money Market Funds [Member] \| Fair Value, Measurements, Recurring [Member]
Cash Equivalents:
Cash equivalents	35,480	25,510
Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Investments:
Investments	35,480	25,510
Level 1 [Member] \| Money Market Funds [Member] \| Fair Value, Measurements, Recurring [Member]
Cash Equivalents:
Cash equivalents	35,480	25,510
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Investments:
Investments	199,719	38,833
Commercial Paper [Member]
Investments:
Investments	12,343	11,957
Commercial Paper [Member] \| Fair Value, Measurements, Recurring [Member]
Cash Equivalents:
Cash equivalents	10,313	4,243
Investments:
Investments	12,343	11,957
Commercial Paper [Member] \| Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Cash Equivalents:
Cash equivalents	10,313	4,243
Investments:
Investments	12,343	11,957
Corporate Bonds [Member]
Investments:
Investments	68,289	17,733
Corporate Bonds [Member] \| Fair Value, Measurements, Recurring [Member]
Cash Equivalents:
Cash equivalents	2,014	4,900
Investments:
Investments	68,289	17,733
Corporate Bonds [Member] \| Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Cash Equivalents:
Cash equivalents	2,014	4,900
Investments:
Investments	68,289	$ 17,733
Certificate of Deposits [Member]
Investments:
Investments	2,272
Certificate of Deposits [Member] \| Fair Value, Measurements, Recurring [Member]
Investments:
Investments	2,272
Certificate of Deposits [Member] \| Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Investments:
Investments	2,272
Government Securities [Member]
Investments:
Investments	104,488
Government Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Investments:
Investments	104,488
Government Securities [Member] \| Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Investments:
Investments	$ 104,488

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair value, assets transfers from Level 1 to Level 2 measurement	$ 0	$ 0
Fair value, assets transfers from Level 2 to Level 1 measurement	0	0
Restricted investments	$ 1,400,000	$ 1,400,000

Investments - Schedule of Investments by Security Type (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 187,439	$ 29,689
Gross Unrealized Gain	14	3
Gross Unrealized Loss	(61)	(2)
Fair Value	187,392	29,690
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,343	11,957
Fair Value	12,343	11,957
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68,333	17,732
Gross Unrealized Gain	8	3
Gross Unrealized Loss	(52)	(2)
Fair Value	68,289	$ 17,733
Certificate of Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,272
Fair Value	2,272
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	104,491
Gross Unrealized Gain	6
Gross Unrealized Loss	(9)
Fair Value	$ 104,488

Investments - Additional Information (Detail) - USD ($)	12 Months Ended
Investments - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Investments [Abstract]
Maximum maturity days for cash equivalents	90 days
Restricted investment	$ 1,400,000	$ 1,400,000
Investments with a contractual maturity over one year	$ 49,825,000	$ 0

Investments - Schedule of Amortized Cost and Fair Value of Investments by Contractual Maturity (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale, amortized cost
Due in 1-year or less	$ 137,588,000
Due after 1-year through 5-years	49,851,000
Amortized Cost	187,439,000	$ 29,689,000
Available-for-sale, fair value
Due in 1-year or less	137,567,000
Due after 1-year through 5-years	49,825,000	0
Fair value	$ 187,392,000	$ 29,690,000

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 48,217	$ 47,237
Less: Accumulated depreciation and amortization	(34,320)	(27,742)
Property and equipment, net	13,897	19,495
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	15,985	15,140
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,874	2,874
Furniture and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,033	1,033
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27,977	27,977
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 348	$ 213

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 6,578	$ 7,603	$ 7,862

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Development and clinical manufacturing costs	$ 6,339	$ 5,605
Payroll and payroll-related costs	6,734	4,609
Facility and other	1,153	670
Total accrued expenses and other current liabilities	$ 14,226	$ 10,884

Leases - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Leased Assets [Line Items]
Existence of option to extend		true
Option to extend, description		Certain leases include one or more options to renew, exercised at the Company’s sole discretion, with renewal terms that can extend the lease from one year to five years.
Cash payments for operating leases		$ 6,302	$ 6,514
Weighted average remaining lease term		2 years 10 months 20 days	3 years 10 months 17 days
Weighted average incremental borrowing rate		11.00%	11.00%
Office, Laboratory and Manufacturing Space [Member]
Operating Leased Assets [Line Items]
Rental expense related to office and laboratory space				$ 4,377
Sublease Agreement [Member]
Operating Leased Assets [Line Items]
Lessee, operating sublease, existence of option to extend	false
Lessee, operating sublease, option to extend, description		The term of the sublease agreement commenced in July 2019 and ends on the last day of the 24th calendar month following commencement, with no option to extend.
Sublease annual rent first year	$ 1,200
Sublease annual rent second year	$ 1,300
Minimum [Member]
Operating Leased Assets [Line Items]
Operating lease, remaining term		1 year
Lease extension period		1 year
Maximum [Member]
Operating Leased Assets [Line Items]
Operating lease, remaining term		3 years
Lease extension period		5 years
As reported under Topic 842 [Member]
Operating Leased Assets [Line Items]
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2019
Change in accounting principle, accounting standards update, transition option elected		us-gaap:AccountingStandardsUpdate201602RetrospectiveMember

Leases - Summary of Future Undiscounted Cash Inflows Under the Sublease (Detail) $ in Thousands	Dec. 31, 2020 USD ($)
Leases [Abstract]
2021	$ 633
Future undiscounted cash inflows	$ 633

Leases - Summary of Operating Lease Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Operating lease assets	$ 9,041	$ 11,356
Liabilities:
Operating lease liabilities	5,115	4,456
Operating lease liabilities, net of current portion	10,561	15,676
Total operating lease liabilities	$ 15,676	$ 20,132

Leases - Summary of Lease Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease costs	$ 4,163	$ 4,532
Short-term lease costs	1,457	1,878
Variable lease costs	2,890	3,022
Sublease income	(1,813)	(890)
Total lease costs	$ 6,697	$ 8,542

Leases - Schedule of Future Payments of Operating Lease Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
2021	$ 6,461
2022	6,390
2023	5,158
Total future payments of operating lease liabilities	18,009
Less: imputed interest	(2,333)
Total operating lease liabilities	$ 15,676	$ 20,132

Restructuring - Additional Information (Detail)		1 Months Ended	12 Months Ended
Restructuring - Additional Information (Detail)	Aug. 10, 2020 Patient	Feb. 28, 2019 Patient	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Restructuring Cost And Reserve [Line Items]
Percent of reduction in headcount of employees		30.00%
Restructuring Charges			$ 0	$ 1,492,000	$ 0
Cash payments			193,000	1,299,000
Severance And Other Termination Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring Charges				1,492,000
Employee Severance And Other Benefits [Member] \| Accrued Expenses and Other Current Liabilities [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring Charges				1,492,000
Cash payments				1,299,000
Outstanding restructuring liabilities			$ 0	$ 193,000
SER-109 [Member] \| Minimum [Member]
Restructuring Cost And Reserve [Line Items]
Number of patients \| Patient	300	300

Restructuring - Schedule of Components of Outstanding Restructuring Liabilities (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost And Reserve [Line Items]
Restructuring expenses	$ 0	$ 1,492,000	$ 0
Cash payments	(193,000)	(1,299,000)
Employee Severance And Other Benefits [Member] \| Accrued Expenses and Other Current Liabilities [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses		1,492,000
Cash payments		(1,299,000)
Liability included in accrued expenses and other current liabilities at December 31, 2019	$ 0	$ 193,000

Notes Payable - Additional Information (Detail) - USD ($)		12 Months Ended
	Oct. 29, 2019	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Interest expense		$ 2,924,000	$ 502,000
Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Interest expense		$ 2,899,000	$ 502,000
Loan and Security Agreement [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility, interest rate	9.65%
Credit facility, payment terms		The Company will make interest only payments through December 1, 2021. The interest only period may be extended to June 1, 2022 upon satisfaction of certain milestones. Following the interest only period, the Company will repay the principal balance and interest of the advances in equal monthly installments through November 1, 2023.
Percentage of prepayment amount during first year	3.00%
Percentage of prepayment amount during second year	2.00%
Percentage of prepayment amount during third year	1.00%
Prepayment or repayment percentage	4.85%
Additional advance prepayment or repayment percentage	4.85%
Carrying value of debt		$ 25,093,000
Loan and Security Agreement [Member] \| Term Loan Facility [Member] \| Current Liability [Member]
Debt Instrument [Line Items]
Carrying value of debt		454,000
Loan and Security Agreement [Member] \| Term Loan Facility [Member] \| Non-current Liability [Member]
Debt Instrument [Line Items]
Carrying value of debt		$ 24,639,000
Loan and Security Agreement [Member] \| Term Loan Facility [Member] \| Prime Rate [Member]
Debt Instrument [Line Items]
Debt instrument, variable rate	4.40%
Hercules Capital, Inc. [Member] \| Loan and Security Agreement [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility, aggregate principal amount	$ 50,000,000
First Tranche [Member] \| Loan and Security Agreement [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Prepayment or repayment charge	1,213,000
Carrying value of debt	$ 24,575,000
Debt instrument, interest rate effective percentage	11.47%
First Tranche [Member] \| Hercules Capital, Inc. [Member] \| Loan and Security Agreement [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Gross proceeds from debt	$ 25,000,000
Second Tranche Unavailable to Borrow Due to Not Met Milestone Requirements [Member] \| Hercules Capital, Inc. [Member] \| Loan and Security Agreement [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility, aggregate principal amount	12,500,000
Third Tranche Available Upon Approval on or Prior to June 30, 2021 [Member] \| Hercules Capital, Inc. [Member] \| Loan and Security Agreement [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility, aggregate principal amount	$ 12,500,000

Notes Payable - Summary of Future Principal Payments Due Under Arrangement, Excluding Interest and End of Term Charge (Detail) $ in Thousands	Dec. 31, 2020 USD ($)
Debt Disclosure [Abstract]
2021	$ 949
2022	11,970
2023	12,081
Total	$ 25,000

Convertible Preferred Stock - Additional Information (Detail) - $ / shares	Dec. 31, 2020	Dec. 31, 2019	Jul. 01, 2015
Equity [Abstract]
Preferred stock, shares authorized	10,000,000	10,000,000	10,000,000
Preferred stock, par value	$ 0.001	$ 0.001	$ 0.001

Stockholders' Equity Common Stock - Additional Information (Detail) $ / shares in Units, $ in Thousands					12 Months Ended
	Aug. 12, 2020 USD ($) $ / shares shares	Nov. 27, 2019 USD ($) $ / shares	Jun. 21, 2019 USD ($) $ / shares shares	Jun. 18, 2019 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Jul. 01, 2015 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized					200,000,000	200,000,000
Common stock, par value \| $ / shares					$ 0.001	$ 0.001
Net proceeds from sale of common stock \| $					$ 243,748	$ 60,527
Underwriting Agreement [Member] \| Cowen and Company, LLC and Piper Sandler & Co. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net proceeds from sale of common stock \| $	$ 243,748
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares issued					12,075,000	28,818,578
Common Stock [Member] \| Underwriting Agreement [Member] \| Cowen and Company, LLC and Piper Sandler & Co. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares issued	10,500,000
Common stock, share price \| $ / shares	$ 21.50
Option exercisable period for purchase additional shares	30 days
Common Stock [Member] \| Maximum [Member] \| Underwriting Agreement [Member] \| Cowen and Company, LLC and Piper Sandler & Co. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option to purchase additional shares of common stock	1,575,000
Initial Public Offering ("IPO") [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized							200,000,000
Common stock, par value \| $ / shares							$ 0.001
Underwritten Public Offering [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net proceeds from sale of common stock \| $	$ 243,748		$ 4,551	$ 55,976
Underwritten Public Offering [Member] \| Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares issued	10,500,000		2,151,911	26,666,667
Common stock, share price \| $ / shares	$ 21.50		$ 2.25	$ 2.25
Underwritten Public Offering [Member] \| Common Stock [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option to purchase additional shares of common stock	1,575,000			2,666,666
At The Market Equity Offering Program [Member] \| 2019 Sales Agreement [Member] \| Cowen And Company, LLC [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, par value \| $ / shares		$ 0.001
Gross proceeds from sale of common stock \| $		$ 25,000
At The Market Equity Offering Program [Member] \| 2019 and 2020 Sales Agreement [Member] \| Cowen And Company, LLC [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares issued					5,787,681
Common stock, share price \| $ / shares					$ 4.40
Net proceeds from sale of common stock \| $					$ 24,773
Percentage of commission on sale of common stock					3
Concurrent Placement [Member] \| Securities Purchase Agreement [Member] \| Nestlé [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares issued	959,002
Common stock, share price \| $ / shares	$ 20.855
Net proceeds from sale of common stock \| $	$ 19,900

Stockholders' Equity Common Stock - 2012 Stock Incentive plan - Additional Information (Detail) - 2012 Stock Incentive Plan [Member]	12 Months Ended
	Dec. 31, 2020 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plan description	The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. The Company generally granted stock-based awards with service conditions only (“service-based” awards).
Shares available for future grant	0
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price per share of common stock expressed as a percentage	100.00%
Stock option granted vesting period	4 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option term	10 years
Stock option granted vesting period	4 years
Stock option expiration period	10 years

Stockholders' Equity Common Stock - 2015 Incentive Award Plan - Additional Information (Detail) - 2015 Incentive Award Plan [Member] - shares		12 Months Ended
	Jun. 16, 2015	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares initially reserved for issuance	2,200,000
Percentage of common stock shares outstanding	4.00%
Stock incentive plan description		The number of shares initially reserved for issuance under the 2015 Plan was the sum of (i) 2,200,000 shares of common stock and (ii) the number of shares subject to awards outstanding under the 2012 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company on or after the effective date of the 2015 Plan. In addition, the number of shares of common stock that may be issued under the 2015 Plan is subject to increase on the first day of each calendar year, beginning in 2016 and ending in 2025, equal to the lesser of (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the preceding applicable calendar year and (ii) an amount determined by the Company’s board of directors.
Shares available for future grant		1,018,232
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period		4 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period		4 years
Stock option expiration period		10 years

Stockholders' Equity Common Stock - 2015 Employee Stock Purchase Plan - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
	Jun. 16, 2015	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 8,824	$ 8,344	$ 16,641
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate shares of common stock reserved for issuance	365,000	2,092,181
Shares issued under ESPP	400,000	155,293
Percentage of common stock shares outstanding	1.00%
Percentage of eligible earning contributed toward semi-annual purchase of common stock		15.00%
Offering period under the plan		6 months
Percentage of purchase price with respect to each offering period		85.00%
Risk-free interest rate		1.20%
Expected term (in years)		6 months
Expected volatility		74.90%
Expected dividend yield		0.00%
Stock-based compensation expense		$ 200	$ 109	$ 145

Stockholders' Equity Common Stock - Summary of Estimated Fair Value of Employee Stock Options (Detail) - Stock Option Valuation [Member]	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.26%	2.64%	2.39%
Expected term (in years)	6 years	6 years	6 years
Expected volatility	73.30%	88.40%	76.00%
Expected dividend yield	0.00%	0.00%	0.00%

Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Equity [Abstract]
Number of Shares, Beginning Balance	8,310,683
Number of Shares, Granted	5,095,365
Number of Shares, Exercised	(2,214,011)
Number of Shares, Forfeited	(1,154,907)
Number of Shares, Ending Balance	10,037,130	8,310,683
Number of Shares, Options exercisable	3,164,129
Weighted Average Exercise Price, Beginning Balance	$ 10.36
Weighted Average Exercise Price, Granted	7.11
Weighted Average Exercise Price, Exercised	6.51
Weighted Average Exercise Price, Forfeited	10.50
Weighted Average Exercise Price, Ending Balance	9.54	$ 10.36
Weighted Average Exercise Price, Options exercisable	$ 14.70
Weighted Average Remaining Contractual Term, Outstanding	7 years 10 months 13 days	7 years 3 days
Weighted Average Remaining Contractual Term, Options exercisable	5 years 9 months
Aggregate Intrinsic Value, Outstanding	$ 156,627	$ 3,427
Aggregate Intrinsic Value, Options exercisable	$ 34,652

Stockholders' Equity Common Stock - Stock Options - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options	$ 5.08	$ 4.13	$ 6.53
Intrinsic Value, Stock options exercised	$ 37,255,000	$ 244,000	$ 1,801,000
Performance-based stock options to granted	5,095,365
Stock based compensation expense for stock options	$ 8,824,000	$ 8,344,000	$ 16,641,000
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options		$ 4.58
Performance-based stock options to granted		1,100,000
Stock options exercisable	0
Stock based compensation expense for stock options	$ 0

Stockholders' Equity Common Stock - Summary of Restricted Stock Activity (Detail) - Restricted Stock Units [Member]	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested restricted stock units, Beginning balance \| shares	130,000
Number of Shares, Granted \| shares	6,500
Number of Shares, Forfeited \| shares	(5,000)
Number of Shares, Vested \| shares	(125,000)
Number of Shares, Unvested restricted stock units, Ending balance \| shares	6,500
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Beginning balance \| $ / shares	$ 8.86
Weighted Average Grant Date Fair Value, Granted \| $ / shares	25.36
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	9.12
Weighted Average Grant Date Fair Value, Vested \| $ / shares	2.29
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Ending balance \| $ / shares	$ 25.36

Stockholders' Equity Common Stock - Restricted Stock Units - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value, vested	$ 532	$ 517	$ 1,206

Stockholders' Equity Common Stock - Summary of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 8,824	$ 8,344	$ 16,641
Research and development expenses [Member] \| Stock Options and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	4,760	4,613	8,388
General and administrative expenses [Member] \| Stock Options and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 4,064	$ 3,731	$ 8,253

Stockholders' Equity Common Stock - Stock Based Compensation - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
Equity [Abstract]
Unrecognized stock- based compensation cost	$ 26,221
Weighted average period over which unrecognized compensation cost is expected to be recognized	3 years 5 months 26 days

Collaboration Revenue - Additional Information (Detail)	1 Months Ended		3 Months Ended				12 Months Ended
Collaboration Revenue - Additional Information (Detail)	Dec. 31, 2020 USD ($)	Mar. 31, 2019 USD ($) Installment	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 30, 2018 USD ($)	Feb. 29, 2016 USD ($)	Jan. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration/license fee							$ 120,000,000
Collaboration revenue - related party							11,897,000	$ 27,188,000	$ 26,917,000
Collaboration revenue							17,161,000	6,215,000
Research Agreement [Member] \| AstraZeneca Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable		$ 20,000,000
Transaction price allocated to remaining performance obligations	$ 23,377,000		$ 23,377,000				$ 23,377,000
Minimum exclusivity period		3 years
Number of installments \| Installment		3
Option exercise period from exclusivity period							90 days
Terms of the definitive license agreement good faith negotiation period							6 months
Other terms of definitive license agreement period							1 year
Termination date	Apr. 02, 2021
Termination notice period	120 days
Research and development fixed consideration	$ 20,000,000		20,000,000				$ 20,000,000
Research and development estimated reimbursement costs							13,900,000
Reimbursement of research and development costs incurred							3,376,000
Collaboration revenue			15,145,000				17,161,000
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	108,174,000		108,174,000				108,174,000	110,071,000					$ 120,000,000
Maximum development milestone payments to be received	285,000,000		285,000,000				285,000,000
Maximum regulatory payments to be received	375,000,000		375,000,000				375,000,000
Maximum amount to be received on achievement of certain commercial milestones	1,125,000,000		1,125,000,000				1,125,000,000
Proceeds on achievement of development milestone							10,000,000		40,000,000	$ 20,000,000	$ 10,000,000
Transaction price allocated to remaining performance obligations	200,000,000		200,000,000		$ 190,000,000		$ 200,000,000		190,000,000
Transaction price of milestone payment to be received				$ 10,000,000		$ 20,000,000
Revenue information used to determine transaction price							The Company estimated the $20,000 of variable consideration by using the most likely amount method which best predicts the amount of consideration to which the Company will be entitled.
Contract modification catch-up adjustment					5,517,000
Collaboration revenue - related party							$ 11,897,000	27,188,000	26,917,000
Cumulative catch-up revenue							4,565,000
Deferred revenue	$ 108,174,000		$ 108,174,000				$ 108,174,000	110,071,000
Nestlé Health Science [Member] \| Phase 2 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable														$ 20,000,000
Nestlé Health Science [Member] \| Phase 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable														$ 20,000,000
Collaboration revenue - related party								$ 6,830,000
Nestlé Health Science [Member] \| Phase 2b [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable					$ 20,000,000				20,000,000			$ 40,000,000
Proceeds on achievement of development milestone									$ 40,000,000
Nestlé Health Science [Member] \| Phase 1b [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue information used to determine transaction price							The Company estimated the $10,000 of variable consideration by using the most likely amount method which best predicts the amount of consideration to which the Company will be entitled.

Collaboration Revenue - Changes in Contract Liabilities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party, Deductions	$ 21,565	$ 27,188
ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party, Balance at beginning of period	110,071	137,259
Deferred revenue - related party, Additions	10,000
Deferred revenue - related party, Deductions	(11,897)	(27,188)
Deferred revenue - related party, Balance at end of period	108,174	110,071
Deferred revenue, Balance at beginning of period	9,668
Deferred revenue, Additions	7,493	15,883
Deferred revenue, Deductions	$ (17,161)	(6,215)
Deferred revenue, Balance at end of period		$ 9,668

Collaboration Revenue - Schedule of Revenue Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$ 21,565	$ 27,188

Net Loss per Share - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss attributable to common stockholders	$ (89,127)	$ (70,279)	$ (98,942)
Denominator:
Weighted average common shares outstanding, basic and diluted	79,789,220	56,649,220	40,743,492
Net loss per share attributable to common stockholders, basic and diluted	$ (1.12)	$ (1.24)	$ (2.43)

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22595-107794

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121326447&loc=d3e1448-109256

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=121326447&loc=d3e1377-109256

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
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-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121326447&loc=SL5780133-109256

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 10: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 31
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8924-108599

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

+ Details

Net Loss per Share - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	10,054,416	8,530,504	7,838,114
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	10,037,130	8,310,683	7,561,719
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	6,500	130,000	226,900
Shares Issuable under Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	10,786	89,821	49,495

Commitments and Contingencies - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Indemnification Agreement [Member]
Other Commitments [Line Items]
Obligations accrued	$ 0	$ 0
Legal Contingencies [Member]
Other Commitments [Line Items]
Obligations accrued	$ 0	$ 0

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Income tax benefit	$ 0	$ 0	$ 0
Unrecognized tax benefits	0	$ 0
Massachusetts [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 386,900,000
Net operating loss carry forwards, expiration year, start	2035
After 2017 and Before January 1, 2021 [Member] \| Maximum [Member]
Income Tax Disclosure [Line Items]
Taxable income on NOL	100.00%
After January 1, 2021 [Member] \| Maximum [Member]
Income Tax Disclosure [Line Items]
Taxable income on NOL	80.00%
Federal [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 390,000,000
Federal [Member] \| Before 2018 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 119,781,000
Net operating loss carry forwards, expiration year, start	2035
Federal [Member] \| After 2017 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 270,219,000
Federal [Member] \| Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 36,400,000
Tax credit carryforwards, expiration year, start	2031
Federal [Member] \| Orphan Drug Credit Carryforwards [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 20,700,000
State [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	386,900,000
State [Member] \| Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 7,500,000
Tax credit carryforwards, expiration year, start	2028

Income Taxes - Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(21.00%)	(21.00%)	(21.00%)
Research and development tax credits	(6.40%)	(5.80%)	(7.20%)
State taxes, net of federal benefit	(7.80%)	(6.80%)	(7.20%)
Stock-based compensation	(5.80%)	(1.70%)	2.00%
Other	0.20%	(0.70%)	0.80%
Change in deferred tax asset valuation allowance	40.80%	36.00%	32.60%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 106,351	$ 72,752
Research and development tax credit carryforwards	42,311	36,602
Capitalized organization costs	214	244
Stock-based compensation expense	11,615	12,783
Lease Liability	4,283	5,500
Charitable Contributions	15	13
Deferred Revenue	29,553	32,713
Accrued expenses	1,783	1,251
Capitalized research and development expenses	51	58
Section 163(j) limitation	540
Total deferred tax assets	196,716	161,916
Deferred tax liabilities:
Depreciation and amortization	(510)	(1,467)
Right of use assets	(2,470)	(3,103)
Total deferred tax liabilities	(2,980)	(4,570)
Valuation allowance	$ (193,736)	$ (157,346)	$ (132,009)	$ (94,126)

Income Taxes - Summary of Changes In Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Valuation Allowance [Abstract]
Valuation allowance at beginning of year	$ (157,346)	$ (132,009)	$ (94,126)
Increases recorded to income tax provision	(36,390)	(25,337)	(37,883)
Valuation allowance as of end of year	$ (193,736)	$ (157,346)	$ (132,009)

Related Party Transactions - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 29, 2016
Related Party Transaction [Line Items]
Collaboration revenue - related party	$ 11,897,000	$ 27,188,000	$ 26,917,000
Sublease income	1,813,000	890,000
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Collaboration revenue - related party	11,897,000	27,188,000	$ 26,917,000
Deferred revenue	108,174,000	110,071,000		$ 120,000,000
Payments under agreements with related party	0
Due from related party for the reimbursement of development costs	$ 0
Flagship Pioneering [Member] \| Sublease Agreement [Member]
Related Party Transaction [Line Items]
Sub lease commencement date	2019-07
Lessee, operating sublease, existence of option to extend	false
Lessee, operating sublease, option to extend, description	The term of the sublease agreement commenced in July 2019 and ends on the last day of the 24th calendar month following commencement, with no option to extend
Cash received from related party transaction	$ 1,813,000	890,000
Flagship Pioneering [Member] \| Sublease Agreement [Member] \| Other Income [Member]
Related Party Transaction [Line Items]
Sublease income	$ 1,813,000	$ 890,000

401(k) Savings Plan - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2016	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Employer match of employee contributions	50.00%
Employee deferral subject to employer match	6.00%
Employer contribution expenses		$ 586	$ 542	$ 604