Seres Therapeutics (MCRB) 10-Q 3 Aug 21 2021 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2021	Jul. 28, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
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
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 Common Stock Shares Outstanding		91,718,571
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, par value $0.001
Security Exchange Name	NASDAQ

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 73,997	$ 116,049
Short term investments	140,556	137,567
Prepaid expenses and other current assets	8,751	5,774
Accounts receivable	1,251	9,387
Total current assets	224,555	268,777
Property and equipment, net	15,047	13,897
Operating lease assets	12,415	9,041
Restricted investments	2,150	1,400
Long term investments	14,879	49,825
Other non-current assets	602
Total assets	269,648	342,940
Current liabilities:
Accounts payable	5,450	4,018
Accrued expenses and other current liabilities	18,913	14,226
Operating lease liabilities	5,675	5,115
Short term portion of note payable, net of discount	6,298	454
Deferred revenue - related party	19,829	22,602
Total current liabilities	56,165	46,415
Long term portion of note payable, net of discount	19,036	24,639
Operating lease liabilities, net of current portion	13,828	10,561
Deferred revenue, net of current portion - related party	78,434	85,572
Other long-term liabilities	1,119	1,003
Total liabilities	168,582	168,190
Commitments and contingencies (Note 10)
Stockholders’ deficit:
Preferred stock, $0.001 par value; 10,000,000 shares authorized at June 30, 2021 and December 31, 2020; no shares issued and outstanding at June 30, 2021 and December 31, 2020
Common stock, $0.001 par value; 200,000,000 shares authorized at June 30, 2021 and December 31, 2020; 91,713,810 and 91,459,239 shares issued and outstanding at June 30, 2021 and December 31, 2020, respectively	92	91
Additional paid-in capital	733,533	723,482
Accumulated other comprehensive income (loss)	12	(47)
Accumulated deficit	(632,571)	(548,776)
Total stockholders’ equity	101,066	174,750
Total liabilities and stockholders’ equity	$ 269,648	$ 342,940

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2021	Dec. 31, 2020
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	91,713,810	91,459,239
Common stock, shares outstanding	91,713,810	91,459,239

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue:
Collaboration revenue - related party	$ 5,263	$ 5,186	$ 9,911	$ 10,648
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Grant revenue		$ 831	$ 1,070	$ 1,570
Collaboration revenue		28		2,016
Total revenue	$ 5,263	6,045	10,981	14,234
Operating expenses:
Research and development expenses	35,954	20,099	65,257	41,842
General and administrative expenses	17,451	6,491	29,192	12,629
Total operating expenses	53,405	26,590	94,449	54,471
Loss from operations	(48,142)	(20,545)	(83,468)	(40,237)
Other (expense) income:
Interest income	829	74	1,795	233
Interest expense	(732)	(719)	(1,428)	(1,435)
Other (expense) income	(285)	476	(694)	844
Total other (expense) income, net	(188)	(169)	(327)	(358)
Net loss	$ (48,330)	$ (20,714)	$ (83,795)	$ (40,595)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.53)	$ (0.28)	$ (0.91)	$ (0.56)
Weighted average common shares outstanding, basic and diluted	91,659,829	73,306,248	91,593,845	72,063,881
Net loss	$ (48,330)	$ (20,714)	$ (83,795)	$ (40,595)
Other comprehensive income:
Unrealized gain on investments, net of tax of $0	27	11	59	1
Total other comprehensive income	27	11	59	1
Comprehensive loss	$ (48,303)	$ (20,703)	$ (83,736)	$ (40,594)

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Unrealized gain on investment, tax	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Statem_3

Condensed Consolidated Statement of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2019	$ (48,324)	$ 70	$ 411,255	$ (459,649)
Beginning balance, shares at Dec. 31, 2019		70,143,252
Issuance of common stock from at the market equity offering, value	4,178	$ 1	4,177
Issuance of common stock from at the market equity offering, shares		1,230,531
Issuance of common stock upon exercise of stock options, value	60	$ 1	59
Issuance of common stock upon exercise of stock options, shares		110,967
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		110,000
Issuance of common stock under ESPP, value	249		249
Issuance of common stock under ESPP, shares		76,317
Stock-based compensation expense	1,959		1,959
Unrealized gain (loss) on investments	(10)				$ (10)
Net loss	(19,881)			(19,881)
Ending balance at Mar. 31, 2020	(61,649)	$ 72	417,819	(479,530)	(10)
Ending balance, shares at Mar. 31, 2020		71,671,067
Beginning balance at Dec. 31, 2019	(48,324)	$ 70	411,255	(459,649)
Beginning balance, shares at Dec. 31, 2019		70,143,252
Net loss	(40,595)
Ending balance at Jun. 30, 2020	(65,575)	$ 75	434,593	(500,244)	1
Ending balance, shares at Jun. 30, 2020		75,131,834
Beginning balance at Mar. 31, 2020	(61,649)	$ 72	417,819	(479,530)	(10)
Beginning balance, shares at Mar. 31, 2020		71,671,067
Issuance of common stock from at the market equity offering, value	14,848	$ 3	14,845
Issuance of common stock from at the market equity offering, shares		3,430,453
Issuance of common stock upon exercise of stock options, value	15		15
Issuance of common stock upon exercise of stock options, shares		30,314
Stock-based compensation expense	1,914		1,914
Unrealized gain (loss) on investments	11				11
Net loss	(20,714)			(20,714)
Ending balance at Jun. 30, 2020	(65,575)	$ 75	434,593	(500,244)	1
Ending balance, shares at Jun. 30, 2020		75,131,834
Beginning balance at Dec. 31, 2020	$ 174,750	$ 91	723,482	(548,776)	(47)
Beginning balance, shares at Dec. 31, 2020	91,459,239	91,459,239
Issuance of common stock upon exercise of stock options, value	$ 372	$ 1	371
Issuance of common stock upon exercise of stock options, shares		104,184
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		650
Issuance of common stock under ESPP, value	392		392
Issuance of common stock under ESPP, shares		24,191
Stock-based compensation expense	3,624		3,624
Unrealized gain (loss) on investments	32				32
Net loss	(35,465)			(35,465)
Ending balance at Mar. 31, 2021	143,705	$ 92	727,869	(584,241)	(15)
Ending balance, shares at Mar. 31, 2021		91,588,264
Beginning balance at Dec. 31, 2020	$ 174,750	$ 91	723,482	(548,776)	(47)
Beginning balance, shares at Dec. 31, 2020	91,459,239	91,459,239
Issuance of common stock upon exercise of stock options, shares	229,730
Net loss	$ (83,795)
Ending balance at Jun. 30, 2021	$ 101,066	$ 92	733,533	(632,571)	12
Ending balance, shares at Jun. 30, 2021	91,713,810	91,713,810
Beginning balance at Mar. 31, 2021	$ 143,705	$ 92	727,869	(584,241)	(15)
Beginning balance, shares at Mar. 31, 2021		91,588,264
Issuance of common stock upon exercise of stock options, value	586		586
Issuance of common stock upon exercise of stock options, shares		125,546
Stock-based compensation expense	5,078		5,078
Unrealized gain (loss) on investments	27				27
Net loss	(48,330)			(48,330)
Ending balance at Jun. 30, 2021	$ 101,066	$ 92	$ 733,533	$ (632,571)	$ 12
Ending balance, shares at Jun. 30, 2021	91,713,810	91,713,810

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities:
Net loss	$ (83,795)	$ (40,595)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,702	3,873
Depreciation and amortization expense	2,902	3,493
Non-cash operating lease cost	1,465	1,100
Accretion of discount on investments	1,581	(102)
Non-cash interest expense	241	215
Changes in operating assets and liabilities:
Prepaid expenses and other current and long-term assets	(3,579)	(1,928)
Accounts receivable	8,136	(1,144)
Deferred revenue	(9,911)	(11,837)
Accounts payable	1,684	(700)
Operating lease liabilities	(1,012)	(2,163)
Accrued expenses and other current and long-term liabilities	4,395	(29)
Net cash used in operating activities	(69,191)	(49,817)
Cash flows from investing activities:
Purchases of property and equipment	(3,896)	(299)
Purchases of investments	(51,942)	(12,931)
Sales and maturities of investments	82,378	40,318
Purchase of restricted investments	(750)
Net cash provided by investing activities	25,790	27,088
Cash flows from financing activities:
Proceeds from exercise of stock options	957	75
Proceeds from issuance of common stock and restricted common stock		120
Proceeds from at the market equity offering, net of commissions		18,695
Issuance of common stock under ESPP	392	249
Net cash provided by financing activities	1,349	19,139
Net decrease in cash and cash equivalents	(42,052)	(3,590)
Cash, cash equivalents and restricted cash at beginning of period	116,049	65,126
Cash, cash equivalents and restricted cash at end of period	73,997	61,536
Supplemental disclosure of cash flow information:
Cash paid for interest	1,220	1,226
Supplemental disclosure of non-cash investing and financing activities:
Unsettled issuance of common stock from the at the market offering		331
Lease liability arising from obtaining right-of-use assets	4,839
Property and equipment purchases included in accounts payable and accrued expenses	$ 606	$ 66

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	6 Months Ended
	Jun. 30, 2021
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 company developing a novel class of live biotherapeutic drugs, which are consortia of microbes designed to treat disease by modulating the microbiome to treat or reduce disease by repairing the function of the microbiome to a non-disease state. The Company’s lead product candidate, SER-109, is designed to reduce further recurrence of Clostridioides difficile 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, therapeutic candidate designed to reduce morbidity and 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. The Company continues to evaluate microbiome pharmacokinetic and pharmacodynamic data from across its clinical and pre-clinical portfolios using its reverse translation microbiome therapeutics capabilities 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 pre-clinical 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, or maintained, that any product candidate developed will obtain necessary government regulatory approval, or that any approved product 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. 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 June 30, 2021, the Company had an accumulated deficit of $ 632,571 and cash, cash equivalents and short and long-term investments of $ 229,432 . For the six months ended June 30, 2021, the Company incurred a net loss of $ 83,795 and used $ 69,191 of cash in operating activities. The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. As of August 3, 2021, the issuance date of the interim consolidated financial statements for the six months ended June 30, 2021, the Company expects that its cash, cash equivalents and short and long-term investments will be sufficient to fund its operating expenses, debt service obligations and capital expenditure requirements 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. (together with NHSc Pharma Partners, “Nestlé”), successor in interest to Nestec Ltd., an affiliate of Nestlé Health Science US Holdings, Inc. (“Nestlé Health Science”), both of which are significant stockholders 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. Unaudited Interim Financial Information The accompanying unaudited condensed consolidated financial statements as of June 30, 2021 and for the three and six months ended June 30, 2021 and 2020 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2020 included in the Company’s annual report on Form 10-K for the fiscal year ended December 31, 2020, which was filed with the SEC on March 2, 2021 (the “Annual Report”). The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited consolidated financial statements. The condensed consolidated balance sheet at December 31, 2020 was derived from audited annual financial statements, but does not contain all of the footnote disclosures from the annual financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position, results of operations, and cash flows for the periods presented. Such adjustments are of a normal and recurring nature. The results of operations for the three and six months ended June 30, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies The significant accounting policies and estimates used in preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2020, and the notes thereto, which are included in the Annual Report. There have been no material changes to the Company’s significant accounting policies during the six months ended June 30, 2021. 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 condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. In the condensed consolidated financial statements, the Company uses estimates and assumptions related to revenue recognition and the accrual of research and development expenses. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. 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. 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 restricted stock units granted by the Company entitle the holder of such awards to ordinary cash dividends paid to substantially all holders of the Company’s common stock, as if such shares were outstanding common shares at the time of the dividend. The dividends are paid in cash or shares of common stock when the applicable restricted stock unit vests. However, the unvested restricted stock units 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. 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: Six Months Ended June 30, 2021 2020 Stock options to purchase common stock 11,687,959 11,578,482 Unvested restricted stock units 461,248 15,000 Shares issuable under ESPP 28,687 25,230 Total common stock equivalents 12,177,894 11,618,712 Recently Issued Accounting Standards 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 Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements 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 June 30, 2021 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 22,428 $ — $ — $ 22,428 Commercial paper — — — — Investments: Commercial paper — 12,694 — 12,694 Corporate bonds — 60,502 — 60,502 Certificate of deposits — 2,272 — 2,272 Government securities — 79,967 — 79,967 $ 22,428 $ 155,435 $ — $ 177,863 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 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, corporate bonds, certificate of deposits and government securities 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 three and six months ended June 30, 2021 and 2020. As of June 30, 2021 and December 31, 2020, the Company held restricted investments of $ 2,150 and $ 1,400 , respectively, which represent certificates of deposit that are classified as Level 2 in the fair value hierarchy.

Investments

Investments	6 Months Ended
	Jun. 30, 2021
Investments Debt And Equity Securities [Abstract]
Investments	4. Investments Investments by security type consisted of the following at June 30, 2021 and December 31, 2020 (in thousands): June 30, 2021 Amortized Gross Gross Fair Investments: Commercial paper $ 12,694 $ — $ — $ 12,694 Corporate bonds 60,499 13 ( 10 ) 60,502 Certificate of deposit 2,272 — — 2,272 Government securities 79,958 11 ( 2 ) 79,967 $ 155,423 $ 24 $ ( 12 ) $ 155,435 December 31, 2020 Amortized Gross Gross Fair 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 Investments with original maturities of less than 90 days are included in cash and cash equivalents on the condensed consolidated balance sheets and are not included in the table above. Investments with maturities of less than 12 months are considered current and those investments with maturities greater than 12 months are considered non-current assets. Excluded from the tables above are restricted investments of $ 2,150 and $ 1,400 as the cost approximates current fair value as of June 30, 2021 and December 31, 2020, respectively. The amortized cost and fair value of investments in commercial paper, corporate bonds, certificate of deposits and government securities by contractual maturity, as of June 30, 2021 and December 31, 2020 were as follows (in thousands): Available-for-Sale as of June 30, 2021 Available-for-Sale as of December 31, 2020 Cost Fair Value Cost Fair Value Due in 1-year or less $ 140,544 $ 140,556 $ 137,588 $ 137,567 Due after 1-year through 5-years 14,879 14,879 49,851 49,825 $ 155,423 $ 155,435 $ 187,439 $ 187,392

Property and Equipment, Net

Property and Equipment, Net	6 Months Ended
	Jun. 30, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): June 30, 2021 December 31, 2020 Laboratory equipment $ 16,903 $ 15,985 Computer equipment 2,977 2,874 Furniture and office equipment 1,033 1,033 Leasehold improvements 27,977 27,977 Construction in progress 3,379 348 52,269 48,217 Less: Accumulated depreciation and amortization ( 37,222 ) ( 34,320 ) $ 15,047 $ 13,897 Depreciation and amortization expense was $ 1,426 , $ 2,902 , $ 1,691 and $ 3,493 for the three and six months ended June 30, 2021 and 2020 , respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	6 Months Ended
	Jun. 30, 2021
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 (in thousands): June 30, 2021 December 31, 2020 Development and clinical manufacturing costs $ 12,225 $ 6,339 Payroll and payroll-related costs 5,203 6,734 Facility and other 1,485 1,153 $ 18,913 $ 14,226

Note Payable

Note Payable	6 Months Ended
	Jun. 30, 2021
Debt Disclosure [Abstract]
Note Payable	7. Note 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 Company elected not to borrow the third tranche of $ 12,500 , which was available upon Hercules’ approval until 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 June 30, 2021, the carrying value of the debt is $ 25,334 , of which $ 6,298 is classified as a current liability and $ 19,036 is classified as a long-term liability on the condensed consolidated balance sheet. As of December 31, 2020, the carrying value of the debt was $ 25,093 , of which $ 454 was classified as a current liability and $ 24,639 was classified as a long-term liability on the condensed consolidated balance sheet. As of June 30, 2021 the future principal payments due under the arrangement, excluding interest and the end of term charge, are as follows (in thousands): Year Ending December 31, Principal 2021 (remaining 6 months) $ 949 2022 11,970 2023 12,081 Total $ 25,000 During the three and six months ended June 30, 2021 and 2020, the Company recognized $ 732 , $ 1,453 , $ 719 and $ 1,435 , respectively, of interest expense related to the Loan Agreement, which is reflected in interest expense on the condensed consolidated statement of operations and comprehensive loss.

Common Stock and Stock-Based Aw

Common Stock and Stock-Based Awards	6 Months Ended
	Jun. 30, 2021
Equity [Abstract]
Common Stock and Stock-Based Awards	8. Common Stock and Stock-Based Awards On May 21, 2021, the Company entered into a Sales Agreement (the “Sales Agreement”) with Cowen and Company, LLC (“Cowen”) to sell shares of the Company’s common stock, with aggregate gross sales proceeds of up to $ 150,000 , from time to time, through an “at the market” equity offering program under which Cowen acts as sales agent. As of June 30, 2021 , the Company had no t sold any shares of common stock under the Sales Agreement. Stock Options The following table summarizes the Company’s stock option activity since December 31, 2020: Number Weighted Weighted Aggregate (in years) Outstanding as of December 31, 2020 10,037,130 $ 9.54 7.87 $ 156,627 Granted 2,617,056 23.06 Exercised ( 229,730 ) 4.16 Forfeited ( 736,497 ) 12.78 Outstanding as of June 30, 2021 11,687,959 $ 12.47 7.82 $ 143,215 Options exercisable as of June 30, 2021 4,886,724 $ 11.01 6.39 $ 66,872 The weighted average grant-date fair value of stock options granted during the three and six months ended June 30, 2021 and 2020 was $ 16.26 , $ 18.51 , $ 2.41 and $ 2.17 per share, respectively. During the three months ended March 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. During the three months ended March 31, 2021, the Company modified the determination date to achieve the specified performance targets for 50 % of the performance-based stock options. The determination date to achieve the specified performance targets was not modified for the remaining 50 % of the performance-based stock options and these options expired on April 1, 2021 . As of June 30, 2021 , none of the 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 June 30, 2021 , the Company did no t record any expense for these performance-based stock options. Additionally, during the three months ended March 31, 2021, the Company granted performance-based stock options to employees for the purchase of an aggregate 440 thousand shares of common stock with a grant date fair value of $ 14.93 per share. These stock options are exercisable only upon achievement of specified performance targets. As of June 30, 2021 , 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 June 30, 2021 , the Company did no t record any expense for these stock options in the three and six months ended June 30, 2021. Restricted Stock Units The Company has granted restricted stock units ("RSUs") with time-based vesting conditions. The table below summarizes the Company’s restricted stock unit activity since December 31, 2020: Number Weighted Unvested restricted stock units as of December 31, 2020 6,500 $ 25.36 Granted 461,053 $ 23.19 Vested ( 650 ) $ 25.36 Forfeited ( 5,655 ) $ 24.04 Unvested restricted stock units as of June 30, 2021 461,248 $ 23.20 The Company has granted RSUs with service-based vesting conditions. RSUs represent the right to receive shares of common stock upon meeting specified vesting requirements. Unvested shares of restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the service-based vesting conditions of each award. During the six months ended June 30, 2021, the Company granted 461,053 RSUs. RSU’s generally vest over four years , with 25 % vesting after one year, and the remaining 75 % vesting quarterly over the next 3 years, subject to continued service to the Company through the applicable vesting date. Stock-based Compensation Expense The Company recorded stock-based compensation expense in the following expense categories of its condensed consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended June 30, Six Months Ended 2021 2020 2021 2020 Research and development expenses $ 2,709 $ 986 $ 4,846 $ 1,990 General and administrative expenses 2,369 928 3,856 1,883 $ 5,078 $ 1,914 $ 8,702 $ 3,873

Collaboration Revenue

Collaboration Revenue	6 Months Ended
	Jun. 30, 2021
Revenue From Contract With Customer [Abstract]
Collaboration Revenue	9. Collaboration Revenue Collaboration and License Agreement with Société des Produits Nestlé S.A. (Nestlé) Summary of Agreement In January 2016, the Company entered into a collaboration and license agreement with Nestlé (the “2016 License Agreement”) for the development and commercialization of certain product candidates for the treatment and management of CDI and inflammatory bowel disease ("IBD"), including UC and Crohn’s disease. The 2016 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 “2016 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 2016 License Agreement, the Company granted to Nestlé an exclusive, royalty-bearing license to develop and commercialize, in the 2016 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 “2016 Collaboration Products”). The 2016 License Agreement sets forth the Company’s and Nestlé’s respective obligations for development, commercialization, regulatory and manufacturing and supply activities for the 2016 Collaboration Products with respect to the licensed fields and the 2016 Licensed Territory. Under the 2016 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 the 2016 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 2016 Collaboration Products in the 2016 Licensed Territory. Under the 2016 License Agreement, the Company is 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 2016 License Agreement to address the current clinical plans for SER-287. Pursuant to the letter agreement, the Company and Nestlé agreed that following initiation of the SER-287 Phase 2b study, the Company would be entitled to receive $ 40,000 in milestone payments from Nestlé, which represent the milestone payments due to the Company for the initiation of a SER-287 Phase 2 study and a Phase 3 study. 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. The 2016 License Agreement continues in effect until terminated by either party on the following bases: (i) Nestlé may terminate the 2016 License Agreement in the event of serious safety issues related to any of the 2016 Collaboration Products; (ii) the Company may terminate the 2016 License Agreement if Nestlé challenges the validity or enforceability of any of the Company’s licensed patents; and (iii) either party may terminate the 2016 License Agreement in the event of the other party’s uncured material breach or insolvency. Upon termination of the 2016 License Agreement, all licenses granted to Nestlé by the Company will terminate, and all rights in and to the 2016 Collaboration Products in the 2016 Licensed Territory will revert to the Company. If the Company commits a material breach of the 2016 License Agreement, Nestlé may elect not to terminate the 2016 License Agreement but instead apply specified adjustments to its payment obligations and other terms and conditions of the 2016 License Agreement. Accounting Analysis The Company assessed the 2016 License Agreement in accordance with ASC 606— Revenue From Contracts with Customers (“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 2016 Collaboration Products in the 2016 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. As of June 30, 2021, the aggregate amount of the transaction price allocated to the remaining performance obligation of the 2016 License Agreement was approximately $ 200,000 . During the three and six months ended June 30, 2021 and 2020, using the cost-to-cost method, which best depicts the transfer of control to the customer, the Company recognized $ 5,263 , $ 9,911 , $ 5,186 and $ 10,648 of collaboration revenue – related party, respectively. As of June 30, 2021 and December 31, 2020, there was $ 98,263 and $ 108,174 , respectively, of deferred revenue related to the unsatisfied portion of the performance obligation under the 2016 License Agreement. As of June 30, 2021, the deferred revenue is classified as current or non-current in the condensed consolidated balance sheets based on the Company’s estimate of revenue that will be recognized within the next 12 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. All costs associated with the 2016 License Agreement are recorded in research and development expense in the condensed 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 conducted 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 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 paid 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 were payable even if the Research Agreement was terminated in accordance with its terms, unless the Research Agreement is terminated by AstraZeneca for the Company’s uncured material breach. Additionally, AstraZeneca would bear its costs of conducting activities under the research plan and would 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 Research Agreement or coming into the control of the Company during the term of the Research 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 exercised an option during the Option Exercise Period, the parties would 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 was reached during the Negotiation Period, subject to certain other terms and conditions applicable for a one ( 1 ) year period, the Company was 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 continued in effect until the Research Agreement was 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 was effective on April 2, 2021 , which was 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) participating 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 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 three and six months ended June 30, 2020, the Company recognized collaboration revenue of $ 28 and $ 2,016 , respectively, under the Research Agreement. No collaboration revenue was recognized for three and six months ended June 30, 2021, as the Company's performance obligations under the Research Agreement ended December 31, 2020. Contract Balances from Contracts with Customers The following table presents changes in the Company’s contract liabilities during the six months ended June 30, 2021 and 2020 (in thousands): Balance as of December 31, 2020 Additions Deductions Balance as of June 30, 2021 Six Months Ended June 30, 2021 Contract liabilities: Deferred revenue - related party $ 108,174 — ( 9,911 ) $ 98,263 Balance as of December 31, 2019 Additions Deductions Balance as of June 30, 2020 Six Months Ended June 30, 2020 Contract liabilities: Deferred revenue - related party $ 110,071 — ( 10,648 ) $ 99,423 Deferred revenue $ 9,668 827 ( 2,016 ) $ 8,479 During the three and six months ended June 30, 2021 and 2020 the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods (in thousands): Three Months Ended Six Months Ended 2021 2020 2021 2020 Revenue recognized in the period from: Amounts included in the contract liability at the beginning of the period $ 5,263 $ 5,203 $ 9,911 $ 11,837 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.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies 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 no t accrued any liabilities related to such obligations in its condensed consolidated financial statements as of June 30, 2021 or December 31, 2020. 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 no t accrue any liabilities related to legal contingencies in its condensed consolidated financial statements as of June 30, 2021 or December 31, 2020 .

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes The Company did no t provide for any income taxes for the three and six months ended June 30, 2021 and 2020. 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 June 30, 2021 and December 31, 2020. Management reevaluates the positive and negative evidence at each reporting period. As of June 30, 2021 and December 31, 2020 , the Company had no accrued interest or tax penalties recorded. 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.

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2021
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions As described in Note 9, in January 2016 the Company entered into the 2016 License Agreement with Nestlé 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 and its affiliate Nestlé Health Science U.S. Holdings, Inc., are significant stockholders. During the three and six months ended June 30, 2021 and 2020, the Company recognized $ 5,263 , $ 9,911 , $ 5,186 and $ 10,648 of related party revenue associated with the 2016 License Agreement, respectively. As of June 30, 2021 and December 31, 2020 there was $ 98,263 and $ 108,174 of deferred revenue related to the 2016 License Agreement, which is classified as current or non-current in the condensed consolidated balance sheets. The Company has made no payments to Nestlé during the three and six months ended June 30, 2021 and 2020 . There is no amount due from Nestlé as of June 30, 2021. In August 2020, the Company entered into the Securities Purchase Agreement with 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. 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 24 th calendar month following commencement, with no option to extend . Under this agreement, the Company recorded other income of $ 464 , $ 925 , $ 449 and $ 899 during the three and six months ended June 30, 2021 and 2020 respectively. The Company received cash payments of $ 464 , $ 925 , $ 449 and $ 899 during the three and six months ended June 30, 2021 and 2020 , respectively.

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent Events License Agreement with NHSc Pharma Partners (Nestlé) On July 1, 2021, the Company entered into a License Agreement (the “2021 License Agreement”) with NHSc Pharma Partners (together with Société des Produits Nestlé S.A., “Nestlé”). Pursuant to the 2021 License Agreement, the Company granted to Nestlé, under certain of the Company’s patent rights and know how, a co-exclusive, sublicensable (under certain circumstances) license to develop, commercialize and conduct medical affairs activities for (i) therapeutic products based on the Company’s microbiome technology (including the Company’s SER-109 product) that are developed by or on behalf of the Company for the treatment of Clostridioides difficile infection (“CDI”) and recurrent CDI, as well as any other indications pursued for the products upon mutual agreement of the parties (the “2021 Field”), in the United States and Canada (the “2021 Licensed Territory”), and (ii) the Company’s SER-109 product and any improvements and modifications thereto developed pursuant to the terms of the 2021 License Agreement (the “2021 Collaboration Products”) for any indications in the 2021 Licensed Territory. The 2021 License Agreement sets forth the parties’ respective obligations for development, regulatory, commercialization, medical affairs, and manufacturing and supply activities for the 2021 Collaboration Products with respect to the 2021 Field and the 2021 Licensed Territory. Pursuant to the 2021 License Agreement, the Company is responsible for, and will use commercially reasonable efforts in, conducting development of SER-109 in the 2021 Field in the United States until first regulatory approval for SER-109 is obtained in the 2021 Field in the United States and in accordance with a development and regulatory activity plan, at the Company’s cost, subject to certain exceptions specified in the 2021 License Agreement. The Company is also responsible for all regulatory affairs related to 2021 Collaboration Products in the 2021 Field in the 2021 Licensed Territory, at its cost, except that expenses incurred for regulatory activities approved by a joint steering committee pursuant to a life cycle management plan for 2021 Collaboration Products are shared equally between the parties. The Company will be solely responsible for manufacturing and supplying 2021 Collaboration Products for development in the 2021 Field in the 2021 Licensed Territory. Nestlé has the sole right to commercialize the 2021 Collaboration Products in the 2021 Licensed Territory in accordance with a commercialization plan, subject to the Company’s right to elect to provide up to a specified percentage of all promotional details for a certain target audience. Each party will use commercially reasonable efforts to commercialize the 2021 Collaboration Products in the 2021 Licensed Territory in accordance with the commercialization plan. Both parties will perform medical affairs activities for 2021 Collaboration Products in the 2021 Licensed Territory in accordance with a medical affairs plan. The Company will be solely responsible for the manufacturing and supply of 2021 Collaboration Products for commercialization under a supply agreement that will be entered into between the parties. The Company will be responsible for commercialization and medical affairs activities costs incurred by the parties until first commercial sale of the first 2021 Collaboration Product in the 2021 Licensed Territory and in accordance with a pre-launch plan, up to a specified cap. Following first commercial sale of the first 2021 Collaboration Product, the Company will be entitled to a royalty in an amount equal to approximately 50 % of the commercial profits. In exchange for the grant of the licenses under the 2021 License Agreement, Nestlé agreed to pay the Company a non-refundable, non-creditable and non-cancellable upfront payment of $ 175.0 million, which was received on July 21, 2021. Nestlé also agreed to pay the Company an additional $ 125.0 million due upon FDA approval of SER-109, $ 10.0 million upon Canadian regulatory approval of SER-109 and sales target milestones payments totaling up to $ 225.0 million. The 2021 License Agreement continues in effect until all development and commercialization activities for all 2021 Collaboration Products in the 2021 Licensed Territory have permanently ceased. The 2021 License Agreement may be terminated by either party upon sixty days ’ written notice for the other party’s material breach that remains uncured during such sixty-day period, or immediately upon written notice for the other party’s insolvency. Nestlé may also terminate the 2021 License Agreement at-will (i) with twelve months’ prior written notice, effective only on or after the third anniversary of first commercial sale of the first 2021 Collaboration Product in the 2021 Licensed Territory, (ii) if first commercial sale of the first 2021 Collaboration Product in the 2021 Licensed Territory has not occurred by the fifth anniversary of the effective date of the 2021 License Agreement, with one hundred eighty days’ prior written notice, which must be provided during a specified period set forth in the 2021 License Agreement, or (iii) if regulatory approval for SER-109 is not granted after submission by the Company of a filing seeking first regulatory approval as set forth in the development and regulatory activity plan, and the parties fail to agree on further development of SER-109 in accordance with the terms of the 2021 License Agreement, with one hundred eighty days’ prior written notice, which must be provided within a specified period set forth in the 2021 License Agreement. The Company may also terminate the 2021 License Agreement immediately upon written notice if Nestlé challenges any licensed patent in the 2021 Licensed Territory. Upon termination of the 2021 License Agreement, all licenses granted to Nestlé by the Company will terminate. If the Company commits a material breach of the 2021 License Agreement, Nestlé may elect not to terminate the 2021 License Agreement but instead apply specified adjustments to the payment terms and other terms and conditions of the 2021 License Agreement. The 2021 License Agreement contains customary representations and warranties by the parties, intellectual property provisions including ownership, patent prosecution, enforcement and defense, certain indemnification rights in favor of each party, and customary confidentiality provisions and limitations of liability. SER-287 Phase 2b Top-line Results On July 22, 2021, the Company announced the topline results of its Phase 2b study evaluating SER-287 in patients with mild-to-moderate UC. The study did not meet its primary endpoint of improving clinical remission rates compared to placebo. Both dosing regimens of SER-287 were generally well tolerated. Given the lack of a clinical efficacy signal identified in SER-287, the Company has decided to close the open label and maintenance portions of the study.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Use of Estimates	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 condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. In the condensed consolidated financial statements, the Company uses estimates and assumptions related to revenue recognition and the accrual of research and development expenses. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. 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.
Net Loss per Share	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 restricted stock units granted by the Company entitle the holder of such awards to ordinary cash dividends paid to substantially all holders of the Company’s common stock, as if such shares were outstanding common shares at the time of the dividend. The dividends are paid in cash or shares of common stock when the applicable restricted stock unit vests. However, the unvested restricted stock units 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. 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: Six Months Ended June 30, 2021 2020 Stock options to purchase common stock 11,687,959 11,578,482 Unvested restricted stock units 461,248 15,000 Shares issuable under ESPP 28,687 25,230 Total common stock equivalents 12,177,894 11,618,712
Recently Issued Accounting Standards	Recently Issued Accounting Standards 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.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
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: Six Months Ended June 30, 2021 2020 Stock options to purchase common stock 11,687,959 11,578,482 Unvested restricted stock units 461,248 15,000 Shares issuable under ESPP 28,687 25,230 Total common stock equivalents 12,177,894 11,618,712

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2021
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 June 30, 2021 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 22,428 $ — $ — $ 22,428 Commercial paper — — — — Investments: Commercial paper — 12,694 — 12,694 Corporate bonds — 60,502 — 60,502 Certificate of deposits — 2,272 — 2,272 Government securities — 79,967 — 79,967 $ 22,428 $ 155,435 $ — $ 177,863 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

Investments (Tables)

Investments (Tables)	6 Months Ended
	Jun. 30, 2021
Investments Debt And Equity Securities [Abstract]
Schedule of Investments by Security Type	Investments by security type consisted of the following at June 30, 2021 and December 31, 2020 (in thousands): June 30, 2021 Amortized Gross Gross Fair Investments: Commercial paper $ 12,694 $ — $ — $ 12,694 Corporate bonds 60,499 13 ( 10 ) 60,502 Certificate of deposit 2,272 — — 2,272 Government securities 79,958 11 ( 2 ) 79,967 $ 155,423 $ 24 $ ( 12 ) $ 155,435 December 31, 2020 Amortized Gross Gross Fair 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
Schedule of Amortized Cost and Fair Value of Investments by Contractual Maturity	The amortized cost and fair value of investments in commercial paper, corporate bonds, certificate of deposits and government securities by contractual maturity, as of June 30, 2021 and December 31, 2020 were as follows (in thousands): Available-for-Sale as of June 30, 2021 Available-for-Sale as of December 31, 2020 Cost Fair Value Cost Fair Value Due in 1-year or less $ 140,544 $ 140,556 $ 137,588 $ 137,567 Due after 1-year through 5-years 14,879 14,879 49,851 49,825 $ 155,423 $ 155,435 $ 187,439 $ 187,392

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	6 Months Ended
	Jun. 30, 2021
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): June 30, 2021 December 31, 2020 Laboratory equipment $ 16,903 $ 15,985 Computer equipment 2,977 2,874 Furniture and office equipment 1,033 1,033 Leasehold improvements 27,977 27,977 Construction in progress 3,379 348 52,269 48,217 Less: Accumulated depreciation and amortization ( 37,222 ) ( 34,320 ) $ 15,047 $ 13,897

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	6 Months Ended
	Jun. 30, 2021
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): June 30, 2021 December 31, 2020 Development and clinical manufacturing costs $ 12,225 $ 6,339 Payroll and payroll-related costs 5,203 6,734 Facility and other 1,485 1,153 $ 18,913 $ 14,226

Note Payable (Tables)

Note Payable (Tables)	6 Months Ended
	Jun. 30, 2021
Debt Disclosure [Abstract]
Summary of Future Principal Payments Due Under Arrangement, Excluding Interest and End of Term Charge	As of June 30, 2021 the future principal payments due under the arrangement, excluding interest and the end of term charge, are as follows (in thousands): Year Ending December 31, Principal 2021 (remaining 6 months) $ 949 2022 11,970 2023 12,081 Total $ 25,000

Common Stock and Stock-Based _2

Common Stock and Stock-Based Awards (Tables)	6 Months Ended
	Jun. 30, 2021
Equity [Abstract]
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2020: Number Weighted Weighted Aggregate (in years) Outstanding as of December 31, 2020 10,037,130 $ 9.54 7.87 $ 156,627 Granted 2,617,056 23.06 Exercised ( 229,730 ) 4.16 Forfeited ( 736,497 ) 12.78 Outstanding as of June 30, 2021 11,687,959 $ 12.47 7.82 $ 143,215 Options exercisable as of June 30, 2021 4,886,724 $ 11.01 6.39 $ 66,872
Summary of Restricted Stock Unit Activity	The Company has granted restricted stock units ("RSUs") with time-based vesting conditions. The table below summarizes the Company’s restricted stock unit activity since December 31, 2020: Number Weighted Unvested restricted stock units as of December 31, 2020 6,500 $ 25.36 Granted 461,053 $ 23.19 Vested ( 650 ) $ 25.36 Forfeited ( 5,655 ) $ 24.04 Unvested restricted stock units as of June 30, 2021 461,248 $ 23.20
Summary of Stock Based Compensation Expense	The Company recorded stock-based compensation expense in the following expense categories of its condensed consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended June 30, Six Months Ended 2021 2020 2021 2020 Research and development expenses $ 2,709 $ 986 $ 4,846 $ 1,990 General and administrative expenses 2,369 928 3,856 1,883 $ 5,078 $ 1,914 $ 8,702 $ 3,873

Collaboration Revenue (Tables)

Collaboration Revenue (Tables)	6 Months Ended
	Jun. 30, 2021
Revenue From Contract With Customer [Abstract]
Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities during the six months ended June 30, 2021 and 2020 (in thousands): Balance as of December 31, 2020 Additions Deductions Balance as of June 30, 2021 Six Months Ended June 30, 2021 Contract liabilities: Deferred revenue - related party $ 108,174 — ( 9,911 ) $ 98,263 Balance as of December 31, 2019 Additions Deductions Balance as of June 30, 2020 Six Months Ended June 30, 2020 Contract liabilities: Deferred revenue - related party $ 110,071 — ( 10,648 ) $ 99,423 Deferred revenue $ 9,668 827 ( 2,016 ) $ 8,479 During the three and six months ended June 30, 2021 and 2020 the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods (in thousands): Three Months Ended Six Months Ended 2021 2020 2021 2020 Revenue recognized in the period from: Amounts included in the contract liability at the beginning of the period $ 5,263 $ 5,203 $ 9,911 $ 11,837

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporated month and year					2010-10
Entity incorporation, state or country code					DE
Accumulated deficit	$ 632,571				$ 632,571		$ 548,776
Cash, cash equivalents and short and long-term investments	229,432				229,432
Net loss	$ 48,330	$ 35,465	$ 20,714	$ 19,881	83,795	$ 40,595
Cash used in operating activities					$ 69,191	$ 49,817

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents	12,177,894	11,618,712
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents	11,687,959	11,578,482
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents	461,248	15,000
Shares Issuable under ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents	28,687	25,230

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Investments:
Investments	$ 155,435	$ 187,392
Fair Value, Measurements, Recurring [Member]
Investments:
Investments	177,863	235,199
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	22,428	35,480
Commercial Paper [Member]
Investments:
Investments	12,694	12,343
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents		10,313
Investments:
Investments	12,694	12,343
Corporate Bonds [Member]
Investments:
Investments	60,502	68,289
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents		2,014
Investments:
Investments	60,502	68,289
Certificates of Deposit
Investments:
Investments	2,272	2,272
Certificates of Deposit | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	2,272	2,272
US Government Agencies Debt Securities
Investments:
Investments	79,967	104,488
US Government Agencies Debt Securities | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	79,967	104,488
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	22,428	35,480
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	22,428	35,480
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	155,435	199,719
Level 2 [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents		10,313
Investments:
Investments	12,694	12,343
Level 2 [Member] | Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents		2,014
Investments:
Investments	60,502	68,289
Level 2 [Member] | Certificates of Deposit | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	2,272	2,272
Level 2 [Member] | US Government Agencies Debt Securities | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	$ 79,967	$ 104,488

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
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	$ 2,150,000	$ 1,400,000

Investments - Schedule of Inves

Investments - Schedule of Investments by Security Type (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 155,423	$ 187,439
Gross Unrealized Gain	24	14
Gross Unrealized Loss	(12)	(61)
Fair Value	155,435	187,392
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,694	12,343
Fair Value	12,694	12,343
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60,499	68,333
Gross Unrealized Gain	13	8
Gross Unrealized Loss	(10)	(52)
Fair Value	60,502	68,289
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,272	2,272
Fair Value	2,272	2,272
US Government Agencies Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	79,958	104,491
Gross Unrealized Gain	11	6
Gross Unrealized Loss	(2)	(9)
Fair Value	$ 79,967	$ 104,488

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Investments [Abstract]
Maximum maturity days for cash equivalents	90 days
Restricted investments	$ 2,150	$ 1,400

Investments - Schedule of Amort

Investments - Schedule of Amortized Cost and Fair Value of Investments by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale, amortized cost
Due in 1-year or less	$ 140,544	$ 137,588
Due after 1-year through 5-years	14,879	49,851
Amortized Cost	155,423	187,439
Available-for-sale, fair value
Due in 1-year or less	140,556	137,567
Due after 1-year through 5-years	14,879	49,825
Fair value	$ 155,435	$ 187,392

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 52,269	$ 48,217
Less: Accumulated depreciation and amortization	(37,222)	(34,320)
Property and equipment, net	15,047	13,897
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	16,903	15,985
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,977	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	$ 3,379	$ 348

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 1,426	$ 1,691	$ 2,902	$ 3,493

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Development and clinical manufacturing costs	$ 12,225	$ 6,339
Payroll and payroll-related costs	5,203	6,734
Facility and other	1,485	1,153
Total accrued expenses and other current liabilities	$ 18,913	$ 14,226

Note Payable - Additional Infor

Note Payable - Additional Information (Detail) - USD ($)	Oct. 29, 2019	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Debt Instrument [Line Items]
Interest expense		$ 732,000	$ 719,000	$ 1,428,000	$ 1,435,000
Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Interest expense		732,000	$ 719,000	$ 1,453,000	$ 1,435,000
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		25,334,000		$ 25,334,000		$ 25,093,000
Loan and Security Agreement [Member] | Term Loan Facility [Member] | Current Liability [Member]
Debt Instrument [Line Items]
Carrying value of debt		6,298,000		6,298,000		454,000
Loan and Security Agreement [Member] | Term Loan Facility [Member] | Non Current Liability [Member]
Debt Instrument [Line Items]
Carrying value of debt		$ 19,036,000		$ 19,036,000		$ 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 Until 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

Note Payable - Summary of Futur

Note Payable - Summary of Future Principal Payments Due Under Arrangement, Excluding Interest and End of Term Charge (Detail) $ in Thousands	Jun. 30, 2021USD ($)
Debt Disclosure [Abstract]
2021 (remaining 6 months)	$ 949
2022	11,970
2023	12,081
Total	$ 25,000

Common Stock and Stock-Based _3

Common Stock and Stock-Based Awards - Additional Information (Details) - USD ($)	May 21, 2021	Jun. 30, 2021	Dec. 31, 2020
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued		91,713,810	91,459,239
At The Market Equity Offering Program [Member] | Sales Agreement [Member] | Cowen And Company, LLC [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued		0
At The Market Equity Offering Program [Member] | Sales Agreement [Member] | Cowen And Company, LLC [Member] | Maximum [Member]
Subsidiary, Sale of Stock [Line Items]
Gross proceeds from sale of common stock	$ 150,000,000

Common Stock and Stock-Based _4

Common Stock and Stock-Based Awards - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
Equity [Abstract]
Number of Shares, Beginning Balance | shares	10,037,130
Number of Shares, Granted | shares	2,617,056
Number of Shares, Exercised | shares	(229,730)
Number of Shares, Forfeited | shares	(736,497)
Number of Shares, Ending Balance | shares	11,687,959	10,037,130
Number of Shares, Options exercisable | shares	4,886,724
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 9.54
Weighted Average Exercise Price, Granted | $ / shares	23.06
Weighted Average Exercise Price, Exercised | $ / shares	4.16
Weighted Average Exercise Price, Forfeited | $ / shares	12.78
Weighted Average Exercise Price, Ending Balance | $ / shares	12.47	$ 9.54
Weighted Average Exercise Price, Options exercisable | $ / shares	$ 11.01
Weighted Average Remaining Contractual Term, Outstanding	7 years 9 months 25 days	7 years 10 months 13 days
Weighted Average Remaining Contractual Term, Options exercisable	6 years 4 months 20 days
Aggregate Intrinsic Value, Outstanding | $	$ 143,215	$ 156,627
Aggregate Intrinsic Value, Options exercisable | $	$ 66,872

Common Stock and Stock-Based _5

Common Stock and Stock-Based Awards - Stock Options - Additional Information (Detail) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2019	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options	$ 16.26		$ 2.41		$ 18.51	$ 2.17
Performance-based stock options to granted					2,617,056
Stock based compensation expense for stock options	$ 5,078,000		$ 1,914,000		$ 8,702,000	$ 3,873,000
Number of Shares, Granted					2,617,056
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options		$ 14.93		$ 4.58
Performance-based stock options to granted		440,000		1,100,000
Percentage of performance-based stock options with modified determination date		50.00%
Percentage of performance-based stock options with no modified determination date		50.00%
Stock options expiration date					Apr. 1, 2021
Stock options exercisable	0				0
Stock based compensation expense for stock options	$ 0				$ 0
Number of Shares, Granted		440,000		1,100,000

Common Stock and Stock-Based _6

Common Stock and Stock-Based Awards - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units [Member]	6 Months Ended
	Jun. 30, 2021$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested restricted stock units, Beginning balance | shares	6,500
Number of Shares, Granted | shares	461,053
Number of Shares, Vested | shares	(650)
Number of Shares, Forfeited | shares	(5,655)
Number of Shares, Unvested restricted stock units, Ending balance | shares	461,248
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Beginning balance | $ / shares	$ 25.36
Weighted Average Grant Date Fair Value, Granted | $ / shares	23.19
Weighted Average Grant Date Fair Value, Vested | $ / shares	25.36
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	24.04
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Ending balance | $ / shares	$ 23.20

Common Stock and Stock-Based _7

Common Stock and Stock-Based Awards - Restricted Stock Units - Additional Information (Detail) - Restricted Stock Units [Member]	6 Months Ended
	Jun. 30, 2021shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	461,053
Vesting period	4 years
Vesting After One Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights percentage	25.00%
Vesting Quarterly Over Next 3 Years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights percentage	75.00%

Common Stock and Stock-Based _8

Common Stock and Stock-Based Awards - Summary of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 5,078	$ 1,914	$ 8,702	$ 3,873
Research and development expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	2,709	986	4,846	1,990
General and administrative expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,369	$ 928	$ 3,856	$ 1,883

Collaboration Revenue - Additio

Collaboration Revenue - Additional Information (Detail)	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 31, 2020USD ($)	Mar. 31, 2019USD ($)Installment	Jun. 30, 2021USD ($)	Jun. 30, 2020USD ($)	Jun. 30, 2021USD ($)	Jun. 30, 2020USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Nov. 30, 2018USD ($)	Feb. 29, 2016USD ($)	Jan. 31, 2016USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration/license fee					$ 120,000,000
Collaboration revenue - related party			$ 5,263,000	$ 5,186,000	$ 9,911,000	$ 10,648,000
Collaboration revenue				28,000		2,016,000
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
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. 2, 2021
Termination notice period	120 days
Research and development fixed consideration	$ 20,000,000		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			0	28,000	0	2,016,000	15,145,000
Nestle Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	108,174,000		98,263,000		98,263,000		108,174,000					$ 120,000,000
Maximum development milestone payments to be received			285,000,000		285,000,000
Maximum regulatory payments to be received			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
Proceeds on achievement of development milestone								$ 40,000,000	$ 20,000,000	$ 10,000,000
Transaction price allocated to remaining performance obligations			200,000,000		200,000,000
Transaction price of milestone payment to be received							10,000,000
Collaboration revenue - related party			5,263,000	$ 5,186,000	9,911,000	$ 10,648,000
Deferred revenue	$ 108,174,000		$ 98,263,000		$ 98,263,000		$ 108,174,000
Nestle Health Science [Member] | Phase 2 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable													$ 20,000,000
Nestle Health Science [Member] | Phase 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable													$ 20,000,000
Nestle Health Science [Member] | Phase 2b [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable											$ 40,000,000
Proceeds on achievement of development milestone								$ 40,000,000

Collaboration Revenue - Changes

Collaboration Revenue - Changes in Contract Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party, Deductions	$ (5,263)	$ (5,203)	$ (9,911)	$ (11,837)
ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party, Balance at beginning of period			108,174	110,071
Deferred revenue - related party, Deductions			(9,911)	(10,648)
Deferred revenue - related party, Balance at end of period	$ 98,263	99,423	$ 98,263	99,423
Deferred revenue, Balance at beginning of period				9,668
Deferred revenue, Additions				827
Deferred revenue, Deductions				(2,016)
Deferred revenue, Balance at end of period		$ 8,479		$ 8,479

Collaboration Revenue - Schedul

Collaboration Revenue - Schedule of Revenue Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$ 5,263	$ 5,203	$ 9,911	$ 11,837

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
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 Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ 0	$ 0	$ 0	$ 0
Accrued interest or tax penalties	$ 0		$ 0		$ 0

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Aug. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Feb. 29, 2016
Related Party Transaction [Line Items]
Collaboration revenue - related party		$ 5,263,000	$ 5,186,000	$ 9,911,000	$ 10,648,000
Nestle Health Science [Member]
Related Party Transaction [Line Items]
Collaboration revenue - related party		5,263,000	5,186,000	9,911,000	10,648,000
Deferred revenue		98,263,000		98,263,000		$ 108,174,000	$ 120,000,000
Payments under agreements with related party		0	0	0	0
Due from related party for the reimbursement of development costs		0		$ 0
Nestlé [Member] | Securities Purchase Agreement [Member] | Concurrent Placement [Member]
Related Party Transaction [Line Items]
Common stock, shares issued	959,002
Common stock, share price	$ 20.855
Aggregate net proceeds from offering of common stock	$ 19,900,000
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		464,000	449,000	$ 925,000	899,000
Flagship Pioneering [Member] | Sublease Agreement [Member] | Other Income [Member]
Related Party Transaction [Line Items]
Sublease income		$ 464,000	$ 449,000	$ 925,000	$ 899,000

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] - 2021 License Agreement [Member] - Nestle Health Science [Member] $ in Millions	Jul. 01, 2021USD ($)
Subsequent Event [Line Items]
Collaboration product, percentage of commercial profit	50.00%
Collaborative arrangement, grant of license for upfront payment	$ 175
Collaborative arrangement, additional payment due upon FDA approval	125
Collaborative arrangement, additional payment due upon Canadian regulatory approval	$ 10
Termination notice period	60 days
Maximum [Member]
Subsequent Event [Line Items]
Collaborative arrangement, sales target milestones payments	$ 225