Seres Therapeutics (MCRB) 10-K 8 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Mar. 02, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	MCRB
Entity Registrant Name	Seres Therapeutics, Inc.
Entity Central Index Key	1,609,809
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		40,648,356
Entity Public Float			$ 229,703,925

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 36,088	$ 54,539
Investments	113,895	138,704
Prepaid expenses and other current assets	5,095	5,126
Total current assets	155,078	198,369
Property and equipment, net	32,931	36,125
Long-term investments		36,752
Restricted cash	1,513	1,400
Total assets	189,522	272,646
Current liabilities:
Accounts payable	7,033	7,587
Accrued expenses and other current liabilities	12,513	10,812
Deferred revenue - related party	12,079	12,058
Total current liabilities	31,625	30,457
Lease incentive obligation, net of current portion	8,989	10,730
Deferred rent	2,233	2,072
Deferred revenue, net of current portion - related party	84,847	96,756
Other long-term liabilities	1,129
Total liabilities	128,823	140,015
Commitments and contingencies (Note 11)
Stockholders’ equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2017 and 2016; no shares issued and outstanding at December 31, 2017 and 2016
Common stock, $0.001 par value; 200,000,000 shares authorized at December 31, 2017 and 2016; 40,571,015 and 40,355,753 shares issued and outstanding at December 31, 2017 and 2016	40	40
Additional paid-in capital	324,376	306,931
Accumulated other comprehensive income (loss)	(146)	(149)
Accumulated deficit	(263,571)	(174,191)
Total stockholders’ equity	60,699	132,631
Total liabilities, convertible preferred stock and stockholders’ equity	$ 189,522	$ 272,646

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
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	40,571,015	40,355,753
Common stock, shares outstanding	40,571,015	40,355,753

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue:
Collaboration revenue - related party	[1]	$ 32,100	$ 21,766
Total revenue		32,100	21,766
Operating expenses:
Research and development expenses		89,455	81,989	$ 38,095
General and administrative expenses		34,040	32,616	16,761
Total operating expenses		123,495	114,605	54,856
Loss from operations		(91,395)	(92,839)	(54,856)
Other income (expense):
Interest income (expense), net		1,590	1,260	83
Other income		425
Revaluation of preferred stock warrant liability				(7)
Total other income (expense), net		2,015	1,260	76
Net loss		$ (89,380)	$ (91,579)	$ (54,780)
Net loss per share attributable to common stockholders, basic and diluted		$ (2.21)	$ (2.30)	$ (2.33)
Weighted average common shares outstanding, basic and diluted		40,449,410	39,846,928	23,532,400
Other comprehensive income (loss):
Unrealized gain (loss) on investments, net of tax of $0		$ 3	$ (179)	$ 30
Total other comprehensive income (loss)		3	(179)	30
Comprehensive loss		$ (89,377)	$ (91,758)	$ (54,750)
[1]	In January 2016, the Company entered into a Collaboration and License Agreement with NHS for the development and commercialization of certain of its product candidates. In exchange for the license, NHS paid the Company an upfront cash payment of $120,000, which the Company received in February 2016 and is being recognized as revenue over the ten year performance period. During the quarter ended September 30, 2016, the Company received $10,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016. During the quarter ended September 30, 2017, the Company received $20,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2017.

Consolidated Statements of Ope5

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Unrealized gain (loss) on investment, tax	$ 0	$ 0	$ 0

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Convertible Preferred Stock Series A, A-2, B, C, D and D-1 [Member]	Series D Convertible Preferred Stock [Member]
Stockholders' (deficit), value, beginning balance at Dec. 31, 2014	$ (26,721)	$ 7	$ 1,104		$ (27,832)
Convertible preferred stock, shares, beginning balance at Dec. 31, 2014						22,866,987
Convertible preferred stock, value, beginning balance at Dec. 31, 2014						$ 136,077
Stockholders' (deficit), shares, beginning balance at Dec. 31, 2014		6,890,250
Issuance of common stock upon exercise of stock options, value	93		93
Issuance of common stock upon exercise of stock options, shares		232,970
Issuance of common stock upon exercise of common stock warrant, value	169	$ 1	168
Issuance of common stock upon exercise of common stock warrant, shares		546,672
Issuance of common stock upon completion of initial public offering, net of offering costs, value	139,267	$ 8	139,259
Issuance of common stock upon completion of initial public offering, net of offering costs, shares		8,545,138
Stock-based compensation expense	9,694		9,694
Issuance costs of convertible preferred stock							$ (24)
Reclassification of preferred stock warrant liability	1,589		1,589
Conversion of convertible preferred stock upon listing, value	136,053	$ 23	136,030			$ (136,053)
Conversion of convertible preferred stock upon listing, shares		22,866,987				(22,866,987)
Unrealized gain (loss) on investments	30			$ 30
Net loss	(54,780)				(54,780)
Stockholders' (deficit), value, ending balance at Dec. 31, 2015	205,394	$ 39	287,937	30	(82,612)
Stockholders' (deficit), shares, ending balance at Dec. 31, 2015		39,082,017
Issuance of common stock upon exercise of stock options, value	2,138	$ 1	2,137
Issuance of common stock upon exercise of stock options, shares		1,273,736
Stock-based compensation expense	16,857		16,857
Unrealized gain (loss) on investments	(179)			(179)
Net loss	(91,579)				(91,579)
Stockholders' (deficit), value, ending balance at Dec. 31, 2016	$ 132,631	$ 40	306,931	(149)	(174,191)
Stockholders' (deficit), shares, ending balance at Dec. 31, 2016	40,355,753	40,355,753
Issuance of common stock upon exercise of stock options, value	$ 116		116
Issuance of common stock upon exercise of stock options, shares	174,386	174,386
Issuance of common stock upon vesting of RSUs	$ (33)		(33)
Issuance of common stock upon vesting of RSUs, shares		40,876
Stock-based compensation expense	17,362		17,362
Unrealized gain (loss) on investments	3			3
Net loss	(89,380)				(89,380)
Stockholders' (deficit), value, ending balance at Dec. 31, 2017	$ 60,699	$ 40	$ 324,376	$ (146)	$ (263,571)
Stockholders' (deficit), shares, ending balance at Dec. 31, 2017	40,571,015	40,571,015

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (89,380)	$ (91,579)	$ (54,780)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	17,362	16,857	9,694
Depreciation and amortization expense	7,259	4,205	728
Loss from revaluation of preferred stock warrant liability			7
Non-cash interest expense	3		367
Accretion of discount on investments	(216)	(386)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	31	(2,598)	(2,470)
Deferred revenue	(11,888)	108,814
Accounts payable	(856)	3,581	2,682
Accrued expenses and other liabilities	2,162	5,027	2,928
Net cash provided by (used in) operating activities	(75,523)	43,921	(40,844)
Cash flows from investing activities:
Purchases of property and equipment	(4,676)	(21,492)	(4,362)
Purchases of investments	(96,534)	(290,594)	(267,761)
Sales and maturities of investments	158,312	246,494	136,390
Changes in restricted cash	(113)	139	(1,400)
Net cash provided by (used in) investing activities	56,989	(65,453)	(137,133)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs			(24)
Proceeds from exercise of stock options and common stock warrants	116	2,138	262
Proceeds from issuance of common stock upon completion of initial public offering			143,015
Payments for repurchase of common stock	(33)
Repayment of notes payable			(2,600)
Payments of initial public offering costs			(2,928)
Net cash provided by financing activities	83	2,138	137,725
Net increase (decrease) in cash and cash equivalents	(18,451)	(19,394)	(40,252)
Cash and cash equivalents at beginning of year	54,539	73,933	114,185
Cash and cash equivalents at end of year	36,088	54,539	73,933
Supplemental disclosure of cash flow information:
Cash paid for interest	1	109	163
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 877	$ 1,650	$ 2,953

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Seres Therapeutics, Inc. (the “Company”) was incorporated under the laws of the State of Delaware in October 2010 under the name Newco LS21, Inc. In October 2011, the Company changed its name to Seres Health, Inc., and in May 2015, the Company changed its name to Seres Therapeutics, Inc. The Company is a microbiome therapeutics platform company developing a novel class of biological drugs, which are designed to restore health by repairing the function of a dysbiotic microbiome. The Company’s lead product candidate, SER-109, is designed to prevent further recurrences of Clostridium difficile infection (“CDI”), a debilitating infection of the colon, and, if approved by the U.S. Food and Drug Administration (“FDA”), could be a first-in-field oral microbiome drug. The Company’s second product candidate, SER-287, is being developed to treat inflammatory bowel disease (“IBD”) including ulcerative colitis (“UC”). The Company is also developing SER-401, a microbiome therapeutic candidate for use with checkpoint inhibitors (CPI’s) in patients with solid tumors. In addition, using its microbiome therapeutics platform, the Company is developing product candidates to treat diseases where the microbiome is implicated, including SER-262, a rationally designed product candidate, to prevent an initial recurrence of primary CDI, SER-301, a rationally designed IBD product candidate, and SER-155, a rationally designed product candidate to prevent infections and improve gastrointestinal barrier function (including the consequences of graft versus host disease) in patients following allogeneic hematopoietic stem cell transplants or solid organ transplants. The Company is also using its microbiome therapeutics platform to conduct research on various indications, including: infectious diseases, metabolic diseases, and inflammatory and immune diseases, including immuno-oncology. 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, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. The Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. The Company has experienced negative cash flows and had an accumulated deficit of $263,571 as of December 31, 2017. For the year ended December 31, 2017, the Company incurred a loss of $89,380 and used $75,523 of cash in operations. The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. The Company expects that its cash, cash equivalents and investments at December 31, 2017 of $149,983 will be sufficient to fund its operating expenses 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 agreement with Nestlé Health Science 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 could constrain the Company’s ability to pursue its business strategies. The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its wholly owned subsidiaries after elimination of all intercompany accounts and transactions.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market accounts, corporate bonds, and certificates of deposit purchased with original maturities of less than 90 days from the date of purchase, are stated at fair value. Restricted Cash The Company held cash of $1,513 as of December 31, 2017 and $1,400 as of December 31, 2016 in a separate restricted bank account as a security deposit for the lease of the Company’s facilities. The Company has classified these deposits as long-term restricted cash on its balance sheet. Investments The Company classifies its available-for-sale investments as current assets on the consolidated balance sheet if they mature within one year from the balance sheet date. The Company classifies all of its investments as available-for-sale securities. The Company’s investments are measured and reported at fair value using quoted prices in active markets for similar securities. Unrealized gains and losses on available-for-sale securities are reported as a separate component of stockholders’ equity (deficit). The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the consolidated statement of operations and comprehensive loss. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary”, the Company reduces the investment to fair value through a charge to the consolidated statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented. Concentration of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. The Company has all cash, cash equivalents and investments balances at accredited financial institutions, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including pre-clinical and clinical testing. These programs could be adversely affected by a significant interruption in the supply of such drug substance products. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above. The Company’s investments in certificates of deposit are carried at amortized cost, which approximates fair value. Certain cash equivalents or investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The carrying values of the Company’s accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities. Deferred Offering Costs The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs (non-current) until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the financing. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset. Laboratory equipment is depreciated over five years. Computer equipment and furniture and office equipment are depreciated over three years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses include salaries, stock-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both pre-clinical studies and clinical trials. Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Accounting for Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options, restricted stock units and restricted stock awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. For stock options or restricted stock units issued with performance-based vesting conditions, the stock compensation expense related to these awards is recognized based on the grant date fair value when achievement of the performance condition is deemed probable. The Company measures stock-based awards granted to consultants and non-employees based on the fair value of the award on the date on which the related service is complete. Compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. The Company recognizes adjustments to stock compensation expense for forfeitures as they occur. The fair value of each stock option grant is estimated on the date of grant using the Black- Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. Revenue Recognition The Company currently generates its revenue through collaboration and license arrangements with strategic partners for the development and commercialization of product candidates. The Company recognizes revenue in accordance with FASB ASC Topic 605, Revenue Recognition (“ASC 605”). Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met: • Persuasive evidence of an arrangement exists • Delivery has occurred or services have been rendered • The seller’s price to the buyer is fixed or determinable • Collectability is reasonably assured Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as long-term deferred revenue. Collaboration Revenue In January 2016 the Company entered into a Collaboration and License Agreement (the “License Agreement”) with Nestec Ltd. (“NHS”), an affiliate of Nestlé Health Science US Holdings, Inc. In connection with the License Agreement, the Company received an upfront, non-refundable payment of $ 120,000 The Company evaluates multiple-element arrangements based on the guidance in FASB ASC Topic 605-25, Revenue Recognition-Multiple-Element Arrangements (“ASC 605-25”). Pursuant to this guidance, the Company identifies the deliverables included in the arrangement and determines: (1) whether the individual deliverables have value to the customer on a standalone basis and represent separate units of accounting or whether they must be accounted for as a combined unit of accounting; and (2) if the arrangement includes a general right of return relative to the delivered item. This evaluation requires management to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that: (i) the delivered item(s) has value to the customer on a standalone basis and (ii) if the arrangement includes a general right of return relative to the delivered item(s), delivery or performance of the undelivered item(s) is considered probable and substantially in the control of the Company. In assessing whether an item has standalone value, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner, the retention of any key rights by the Company, and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use the other deliverable(s) for their intended purpose without the receipt of the remaining element(s), whether the value of the deliverable is dependent on the undelivered item(s) and whether there are other vendors that can provide the undelivered element(s). In situations where the Company has identified multiple units of accounting, the arrangement consideration that is fixed or determinable is allocated among the separate units of accounting using the relative selling price method. The Company determines the selling price of a unit of accounting following the hierarchy of evidence prescribed by ASC 605-25. Accordingly, the Company determines the estimated selling price for units of accounting within each arrangement using vendor-specific objective evidence (“VSOE”) of selling price, if available, third-party evidence (“TPE”) of selling price if VSOE is not available, or best estimate of selling price (“BESP”) if neither VSOE nor TPE is available. Then, the applicable revenue recognition criteria in ASC 605-25 are applied to each of the separate units of accounting to determine the appropriate period and pattern of recognition. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605-25 are satisfied for that particular unit of accounting. The Company will recognize as revenue, upon delivery, arrangement consideration attributed to licenses that have standalone value from the other deliverables to be provided in an arrangement. For licenses that do not have standalone value from the other deliverables to be provided in an arrangement over the Company’s estimated performance period as the arrangement would be accounted for as a single unit of accounting. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605-25 are satisfied for that particular unit of accounting. The Company will recognize as revenue arrangement consideration attributed to licenses that have standalone value from the other deliverables to be provided in an arrangement upon delivery. The Company will recognize as revenue arrangement consideration attributed to licenses that do not have standalone value from the other deliverables to be provided in an arrangement over the Company’s estimated performance period as the arrangement would be accounted for as a single unit of accounting. If there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement for the single unit of accounting on a straight-line basis over the period the Company is expected to complete its performance obligations. Alternatively, if the pattern of performance in which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight-line method or proportional performance method, as applicable. At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the respective milestone and the level of effort and investment required to achieve the respective milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all of the criteria required to conclude that a milestone is substantive. The Company recognizes revenue associated with substantive milestones in accordance with FASB ASC Topic 605-28, Revenue Recognition-Milestone Method upon successful accomplishment of each milestone, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive would be recognized as revenue over the remaining period of performance, assuming all other revenue recognition criteria are met. The Company will recognize royalty revenue in the period of sale of the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met. Refer to footnote 9 for further information related to the Company’s collaboration and license agreement with Nestec, Ltd. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Warrant to Purchase Convertible Preferred Stock The Company classified a warrant to purchase shares of its Series A-2 convertible preferred stock as a liability on its consolidated balance sheets as this warrant was a free-standing financial instrument that could have required the Company to transfer assets upon exercise. The warrant was initially recorded at fair value on date of grant, and it was subsequently remeasured to fair value at each balance sheet date. Changes in fair value of the warrant were recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. The Company continued to adjust the liability for changes in fair value until the warrant became a warrant to purchase common stock in connection with the IPO. The Company used the Black-Scholes option-pricing model, which incorporates assumptions and estimates, to value the preferred stock warrant. The Company assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. Estimates and assumptions impacting the fair value measurement included the fair value per share of the underlying Series A-2 convertible preferred stock, the remaining contractual term of the warrant, the risk-free interest rate, the expected dividend yield and the expected volatility of the price of the underlying preferred stock. The Company determined the fair value per share of the underlying preferred stock by taking into consideration the most recent sales of its convertible preferred stock, results obtained from third-party valuations and additional factors that were deemed relevant. The Company historically has been a private company and lacks company-specific historical and implied volatility information of its stock. Therefore, it estimated its expected stock volatility based on the historical volatility of publicly traded peer companies for a term equal to the remaining contractual term of the warrant. The risk-free interest rate was determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrant. Expected dividend yield was based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing Ecobiotic microbiome therapeutics to treat dysbiosis in the colonic microbiome. Revenue to date has been generated solely through the Company's collaboration with Nestle Health Science, all of which has been earned in the United States. All tangible assets are held in the United States Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2017, 2016 and 2015, other comprehensive income (loss) consisted of changes in unrealized gains (losses) from available-for-sale investments. Net Loss per Share Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options and warrants and unvested restricted stock. The Company applies the two-class method to calculate its basic and diluted net loss per share attributable to common stockholders, as its convertible preferred stock, which converted to Common Stock upon completion of the listing of the Company’s common stock on NASDAQ on June 26, 2015, and common stock are participating securities. The two-class method is an earnings allocation formula that treats a participating security as having rights to earnings that otherwise would have been available to common stockholders. However, the two-class method does not impact the net loss per share of common stock as the Company was in a net loss position for each of the periods presented and preferred stockholders do not participate in losses. The Company’s convertible preferred stock contractually entitles the holders of such shares to participate in dividends but does not contractually require the holders of such shares to participate in losses of the Company. Similarly, restricted stock awards granted by the Company entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding common shares at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Recently Issued and Adopted Accounting Pronouncements In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers The Company adopted ASC 606 using the modified retrospective transition method. The adoption of ASC 606 changed the pattern and timing of revenue recognition of amounts from the Company’s collaboration agreement with NHS. Under ASC 606, the Company will recognize revenue using the cost-to-cost method which best depicts the transfer of control to the customer. Under the cost-to-cost method, the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the identified performance obligation. Under this method, revenue will be recorded as a percentage of the estimated transaction price based on the extent of progress towards completion. The estimate of the Company’s measure of progress and estimate of transaction price will be updated at each reporting date, as a change in estimate, and will require judgement. The amount of consideration allocated to satisfied performance obligations, based on the Company’s measure of progress will be recognized immediately on a cumulative catch-up basis, resulting in an adjustment to revenue in the period of change. The amount related to the unsatisfied portion will be recognized as that portion is satisfied over time. In addition, substantive development milestones, which were previously recognized in the period when the milestone was achieved, will be recognized over the remaining performance period under ASC 606. As the adoption method does not result in a recast of the prior year consolidated financial statements, ASC 606 requires the Company to provide additional disclosures during the year of adoption of the amount by which each financial statement line item is affected by adoption of the new standard and explanations of the reasons for significant changes. During the fourth quarter of 2017, the Company substantially completed its impact assessment, and expects that the adoption of ASC 606 will require a cumulative-effect adjustment of approximately $25,000 In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842) In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU 2016-18, Restricted Cash In May 2017, the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s assets and liabilities as of December 31, 2017 and 2016 that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of December 31, 2017 Using: Level 1 Level 2 Level 3 Not Leveling (1) Total Assets: Cash Equivalents $ — $ — $ — $ 25,964 $ 25,964 Investments: Commercial Paper — 6,198 — — 6,198 Certificates of Deposit — 8,916 — — 8,916 Corporate Bonds — 58,865 — — 58,865 Government Securities — 22,954 — — 22,954 Treasury Bonds — 16,962 — — 16,962 $ — $ 113,895 $ — $ 25,964 $ 139,859 (1) Fair Value Measurements as of December 31, 2016 Using: Level 1 Level 2 Level 3 Not Subject to Leveling (1) Total Assets: Cash Equivalents $ — $ 4,740 $ — $ 1,567 $ 6,307 Repurchase Agreements — 7,000 — — 7,000 Investments: Commercial Paper — 19,689 — — 19,689 Certificates of Deposit — 10,629 — — 10,629 Corporate Bonds — 94,609 — — 94,609 Government Securities — 33,466 — — 33,466 Treasury Bonds — 17,063 — — 17,063 $ — $ 187,196 $ — $ 1,567 $ 188,763 (1) As of December 31, 2017, the Company’s cash equivalents consisted of money market funds with original maturities of less than 90 days from the date of purchase. As of December 31, 2016, the Company’s cash equivalents consisted of money market funds, corporate bonds, certificates of deposits, and repurchase agreements with original maturities of less than 90 days from the date of purchase and were valued based on Level 2 inputs. Repurchase agreements are agreements with banks to repurchase notes that are collateralized by U.S. government securities. All repurchase agreements have overnight maturities. The fair value of the Company’s investments, which consisted of corporate bonds, commercial paper, certificates of deposit, government securities, and treasury bonds as of December 31, 2017 and 2016, were determined using Level 2 inputs. During the years ended December 31, 2017 and 2016, there were no transfers between Level 1, Level 2 and Level 3.

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Investments	4. Investments As of December 31, 2017 and 2016, the fair value of available-for-sale investments by type of security was as follows: December 31, 2017 Amortized Cost Gross Unrealized Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 6,198 $ — $ — $ 6,198 Certificates of Deposit 8,916 — — 8,916 Corporate Bonds 58,937 — (72 ) 58,865 Government Securities 22,997 — (43 ) 22,954 Treasury Bonds 16,992 — (30 ) 16,962 $ 114,040 $ — $ (145 ) $ 113,895 December 31, 2016 Amortized Cost Gross Unrealized Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 19,631 $ 58 $ — $ 19,689 Certificates of Deposit 10,629 — — 10,629 Corporate Bonds 94,764 — (155 ) 94,609 Government Securities 33,513 — (47 ) 33,466 Treasury Bonds 17,066 1 (4 ) 17,063 $ 175,603 $ 59 $ (206 ) $ 175,456 Investments with original maturities of less than 90 days are included in cash and cash equivalents on the consolidated balance sheets and are not included in the table above. Investments with maturities of less than 12 months are considered current and those investments with maturities greater than 12 months are considered non-current. As of December 31, 2017, the Company’s commercial paper, certificates of deposit, corporate bonds, government securities and treasury bonds had remaining maturities of less than 12 months. All investments with unrealized losses at December 31, 2017 have been in a loss position for less than twelve months or the loss is not material and temporary in nature. The Company does not intend to sell the investments that are in an unrealized loss position before recovery of their amortized cost basis.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following: December 31, 2017 December 31, 2016 Laboratory equipment $ 13,181 $ 10,711 Computer equipment 2,832 1,335 Furniture and office equipment 1,033 1,010 Leasehold improvements 27,963 27,807 Construction in progress 361 442 45,370 41,305 Less: Accumulated depreciation and amortization (12,439 ) (5,180 ) $ 32,931 $ 36,125 Depreciation and amortization expense was $7,259, $4,205 and $728 for the years ended December 31, 2017, 2016 and 2015, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	6. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: December 31, 2017 December 31, 2016 Development and clinical manufacturing costs $ 3,910 $ 3,350 Payroll and payroll-related costs 4,962 3,698 Professional fees 344 448 Facility and other 3,297 3,316 $ 12,513 $ 10,812

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Convertible Preferred Stock	7 . Convertible Preferred Stock On July 1, 2015, in connection with the closing of the IPO, the Company effected its Restated Certificate of Incorporation, which authorizes the Company to issue 10,000,000 shares of preferred stock, $0.001 par value per share.

Stockholders_ Equity Common Sto

Stockholders’ Equity Common Stock	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Stockholders’ Equity Common Stock	8. Stockholders’ Equity Common Stock On July 1, 2015, the Company completed an IPO, and issued and sold 8,545,138 shares of common stock at a public offering price of $18.00 per share, resulting in net proceeds of approximately $139,267 after deducting underwriting discounts and commissions and other offering expenses totaling $3,748. The shares issued upon closing of the IPO included 1,114,583 shares of the Company’s common stock, which were sold to the underwriters pursuant to the full exercise of their option to purchase additional shares of common stock. Upon the listing of the Company’s common stock on NASDAQ on June 26, 2015, all outstanding shares of the Company’s convertible preferred stock automatically converted into 22,866,987 shares of the Company’s common stock. As of December 31, 2014, the Company’s Amended and Restated Certificate of Incorporation, as further amended, authorized the Company to issue 38,000,000 shares of common stock, $0.001 par value per share. On July 1, 2015, in connection with the closing of the IPO, the Company effected its Restated Certificate of Incorporation, which authorizes the Company to issue 200,000,000 shares of common stock, $0.001 par value per share. 2012 Stock Incentive Plan The Company’s 2012 Stock Incentive Plan, as amended, (the “2012 Plan”) provides for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. The 2012 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. The Company generally grants stock-based awards with service conditions only (“service-based” awards). Stock options granted under the 2012 Plan generally vest over four years and expire after ten years, although options have been granted with vesting terms less than four years. As of December 31, 2017, there were no shares available for future grant under the 2012 Plan. 2015 Incentive Award Plan On June 16, 2015, the Company’s stockholders approved the 2015 Incentive Award Plan (the “2015 Plan”), which became effective on June 25, 2015. The 2015 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2015 Plan is the sum of (i) 2,200,000 shares of common stock and (ii) the number of shares subject to awards outstanding under the 2012 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company on or after the effective date of the 2015 Plan. In addition, the number of shares of common stock that may be issued under the 2015 Plan is subject to increase on the first day of each calendar year, beginning in 2016 and ending in 2025, equal to the lesser of (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the preceding applicable calendar year and (ii) an amount determined by the Company’s board of directors. As of December 31, 2017, there were 1,450,792 shares available for future grant under the 2015 Plan. 2015 Employee Stock Purchase Plan On June 16, 2015, the Company’s stockholders approved the 2015 Employee Stock Purchase Plan (the “ESPP”), which became effective on June 25, 2015. A total of 365,000 shares of common stock were reserved for issuance under the ESPP. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2016 and ending in 2025, by an amount equal to the least of (i) 400,000 shares, (ii) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (iii) an amount determined by the Company’s board of directors. Offering periods under the ESPP will commence when determined by the plan administrator. As of December 31, 2017, there were no shares issued under the ESPP. Stock Option Valuation The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis: Year Ended December 31, 2017 2016 2015 Risk-free interest rate 2.20 % 1.54 % 1.80 % Expected term (in years) 6.0 6.0 6.0 Expected volatility 80.9 % 84.2 % 81.4 % Expected dividend yield 0 % 0 % 0 % Stock Options The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2017: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2016 5,069,133 $ 14.36 8.26 $ 16,736 Granted 1,749,000 10.21 Exercised (174,386 ) 0.67 Forfeited (518,055 ) 21.07 Outstanding as of December 31, 2017 6,125,692 $ 13.00 7.70 $ 15,808 Options vested and expected to vest as of December 31, 2017 6,125,692 $ 13.00 7.70 $ 15,808 Options exercisable as of December 31, 2017 3,235,166 $ 11.37 7.00 $ 14,074 The weighted average grant-date fair value of stock options granted during the years ended December 31, 2017, 2016 and 2015 was $7.07, $18.45, and $14.56 per share, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2017, 2016, and 2015 was $1,912, $34,871, and $4,125. The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The Company granted performance-based stock options to certain employees. These stock options were exercisable only upon achievement of specified performance targets. During the year ended December 31, 2016, the Company determined that the achievement of specified performance targets occurred. Upon achievement of the performance targets, 60,000 options became immediately vested. The Company recorded stock based compensation expense of $235 during the year ended December 31, 2016 related to these awards. The grant date fair value of these awards was $3.92 per share. As of December 31, 2017and 2016, there were outstanding unvested service-based stock options held by non-employees for the purchase of 45,000 and 63,375 shares, respectively, of common stock. Restricted Stock The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2017: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted stock units as of December 31, 2016 115,500 $ 9.78 Granted 332,500 $ 10.08 Forfeited (47,100 ) $ 9.93 Vested (44,122 ) $ 10.02 Unvested restricted stock units as of December 31, 2017 356,778 $ 10.01 The aggregate intrinsic value of restricted stock units that vested during the years ended December 31, 2017, 2016 and 2015 was $445, $0, and $1,508, respectively. Stock-based Compensation The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its consolidated statements of operations and comprehensive loss: Year Ended December 31, 2017 2016 2015 Research and development expenses $ 8,115 $ 8,310 $ 5,297 General and administrative expenses 9,247 8,547 4,397 $ 17,362 $ 16,857 $ 9,694 As of December 31, 2017, the Company had an aggregate of $28,102 of unrecognized stock- based compensation cost, which is expected to be recognized over a weighted average period of 2.14 years.

Collaboration Revenue

Collaboration Revenue	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaboration Revenue	9. Collaboration Revenue Nestec Ltd. In January 2016, the Company entered into the Collaboration and License Agreement (“License Agreement”) with Nestec Ltd. (“NHS”), an affiliate of Nestlé Health Science US Holdings, Inc., a significant stockholder of the Company, for the development and commercialization of certain product candidates in development for the treatment and management of CDI and IBD, including UC and Crohn’s disease. The License Agreement will support the development of the Company’s portfolio of products for CDI and IBD in markets outside of the United States and Canada (the “Licensed Territory”). The Company has retained full commercial rights to its entire portfolio of product candidates with respect to the United States and Canada. Under the License Agreement, the Company granted to NHS an exclusive, royalty-bearing license to develop and commercialize, in the Licensed Territory, certain products based on its microbiome technology that are being developed for the treatment of CDI and IBD, including SER-109, SER-262, SER-287 and SER-301 (collectively, the “NHS Collaboration Products”). The License Agreement sets forth the Company’s and NHS’ respective obligations for development, commercialization, regulatory and manufacturing and supply activities for the NHS Collaboration Products with respect to the licensed fields and the Licensed Territory. In exchange for the license, NHS agreed to pay the Company an upfront cash payment of $120,000, which the Company received in February 2016. NHS also agreed to pay the Company tiered royalties, at percentages ranging from the high single digits to high teens, of net sales of NHS Collaboration Products in the Licensed Territory. 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 NHS Collaboration Products. At the inception of the License Agreement, the Company identified the following deliverables: (i) a license to develop and commercialize the NHS Collaboration Products in the Licensed Territory, (ii) obligation to perform research and development services, (iii) participation on a joint steering committee, and (iv) manufacturing services to provide clinical supply to complete future clinical trials. The Company also identified a contingent deliverable, the obligation to perform manufacturing services to provide commercial supply if commercialization occurs, which is contingent upon regulatory approval. This contingent deliverable has been excluded from the initial allocation and will be treated as a separate unit of accounting when and if delivered. The Company concluded that none of the four deliverables identified at the inception of the License Agreement has standalone value from the other undelivered elements. Accordingly, all deliverables represent a single unit of accounting. All consideration received relating to the four identified deliverables that comprise the single unit of accounting will be recognized over the period of performance. The period of performance will be through the completion of development services for the NHS Collaboration Products which has been estimated to be ten years. The Company will periodically review and, if necessary, revise the estimated development period. The Company will recognize revenue utilizing a time-based proportional performance model where revenue related to each payment is recognized over the ten-year performance period. As of December 31, 2017, the only consideration that is fixed and determinable is the non-refundable upfront payment of $120,000 and $793 for the reimbursement of development services since the inception of the arrangement. For additional consideration that could be received for research and development services and/or manufacturing services for clinical supply, the Company will recognize a cumulative catch-up for the amount of time that has elapsed and spread the unrecognized portion over the remaining performance period. Development and regulatory milestones that involve substantial effort on the Company’s part and the achievement of which are not considered probable at the inception of the License Agreement are considered substantive milestones, and will be recognized in their entirety in the period in which the milestone is achieved, assuming all other revenue recognition criteria are met. All commercial milestones will be recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met. During the year ended December 31, 2016, the Company received $10,000 from NHS in connection with the initiation of the Phase 1b study for SER-262 in CDI. The Company recognizes revenue associated with substantive milestones in accordance with FASB ASC Topic 605-28, Revenue Recognition-Milestone Method During the year ended December 31, 2017, the Company received $20,000 from NHS in connection with the initiation of the Phase 3 study for SER-109. The Company recognizes revenue associated with substantive milestones in accordance with FASB ASC Topic 605-28, Revenue Recognition-Milestone Method. The $20,000 was recognized in full as related party collaboration revenue during the year ended December 31, 2017. Royalties will be recorded as revenue in the period they are earned assuming all other revenue recognition criteria are met. During the years ended December 31, 2017 and 2016, the Company recognized $32,100 and $21,766, respectively, of related party revenue associated with the License Agreement. As of December 31, 2017, there was $96,926 of deferred revenue related to the License Agreement, which is classified as current or non-current in the consolidated balance sheets based on the Company’s estimate of revenue that will be recognized within the next twelve months. All costs associated with the License Agreement are recorded in research and development expense in the consolidated statements of operations and comprehensive loss.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Loss per Share	10 . Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, 2017 2016 2015 Numerator: Net loss attributable to common stockholders $ (89,380 ) $ (91,579 ) $ (54,780 ) Denominator: Weighted average common shares outstanding, basic and diluted 40,449,410 39,846,928 23,532,400 Net loss per share attributable to common stockholders, basic and diluted $ (2.21 ) $ (2.30 ) $ (2.33 ) The Company’s potential dilutive securities, which include stock options and unvested restricted common stock, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The following potential common shares, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2017 2016 2015 Stock options to purchase common stock 6,125,692 5,069,133 5,026,246 Unvested restricted stock units 356,778 115,500 — 6,482,470 5,184,633 5,026,246

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11 . Commitments and Contingencies Leases The Company previously leased office and laboratory space with a lease term expiring in January 2018 and no extension periods. In May 2016, upon mutual agreement with the landlord, the Company accelerated the termination of the operating lease to June 30, 2016. Upon termination of the lease, the Company recorded a benefit to rent expense of $136 to write off amounts previously recorded as deferred rent. The outstanding security deposit of $119, which was secured by a cash collateralized letter of credit, was released in September 2016. On April 1, 2015, the Company entered into a lease for additional office and laboratory space with a term expiring in April 2020. On November 11, 2015, the Company entered into a non-cancelable property lease with BMR-Sidney Research Campus LLC (“BMR”) for 83,396 square feet of office, laboratory and pilot manufacturing space at 200 Sidney Street, Cambridge, Massachusetts. The lease term commenced in March 2016 and ends in November 2023. The Company has the option to extend the lease twice, each for a five-year period. The Company moved its corporate headquarters to this location in April 2016. BMR has contributed a total of $12,509 toward the cost of tenant improvements. BMR’s contributions toward the cost of tenant improvements is recorded as a lease incentive obligation on the Company’s consolidated balance sheet. The lease incentive obligation is amortized to the Company’s consolidated statement of operations as reductions to rent expense over the lease term. As of December 31, 2017, the Company recorded a lease incentive obligation of $10,312. During the years ended December 31, 2017, 2016 and 2015, the Company recognized $4,458, $3,826, and $1,246, respectively, of rental expense related to office and laboratory space. Future minimum lease payments for these operating leases as of December 31, 2017 are as follows: Year Ending December 31, 2018 $ 6,157 2019 6,342 2020 6,120 2021 6,221 2022 6,372 2023 and thereafter 5,158 Total $ 36,370 Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2017 or 2016. Legal Proceedings On September 28, 2016, a purported stockholder of the Company filed a putative class action lawsuit in the U.S. District Court for the District of Massachusetts against the Company entitled Mariusz Mazurek v. Seres Therapeutics, Inc., et.al. On February 12, 2017, the Company received an amended complaint and on March 30, 2017, the Company filed a motion to dismiss. A hearing on the motion to dismiss was held on August 9, 2017 and we are awaiting a decision from the court. The lawsuit alleges violations of Sections 10(b), 20(a) and Rule 10b-5 of the Securities Exchange Act of 1934, as amended, by making allegedly false and misleading statements and omissions about the Company’s clinical trials for its product candidate SER-109 in the Company’s public disclosures between June 25, 2015 and July 29, 2016. The lawsuit seeks, among other things, damages in connection with the Company’s allegedly inflated stock price between June 25, 2015 and July 29, 2016 as a result of those allegedly false and misleading statements, as well as interest, attorneys’ fees and costs. The Company can make no assurances as to the time or resources that will need to be devoted to this lawsuit or its final outcome, or the impact, if any, of this lawsuit or any proceedings on its business, financial condition, results of operations and cash flows. While the Company is vigorously defending against all claims asserted, this litigation could result in substantial costs to the Company and a diversion of the Company’s management’s attention and resources, which could harm its business. In addition, the uncertainty of the pending lawsuit or potential filing of additional lawsuits could lead to more volatility and a reduction in the Company’s stock price. Given the early stage of the litigation, at this time the Company is unable to reasonably estimate possible losses or form a judgment that an unfavorable outcome is either probable or remote. It is not currently possible to assess whether or not the outcome of these proceedings may have a material adverse effect on the Company.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	12 . Income Taxes During the years ended December 31, 2017, 2016 and 2015, the Company recorded no income tax benefits for the net operating losses incurred in each year or interim period, due to its uncertainty of realizing a benefit from those items. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2017 2016 2015 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% Research and development tax credits (7.7 ) (8.8 ) (5.7 ) State taxes, net of federal benefit (4.8 ) (5.8 ) (5.3 ) Stock-based compensation 1.2 (6.3 ) 4.1 Revaluation of preferred stock warrant liability 33.1 — — Other (0.3 ) 0.1 0.3 Change in deferred tax asset valuation allowance 12.5 54.8 40.6 Effective income tax rate — % — % — % Net deferred tax assets as of December 31, 2017 and 2016 consisted of the following: December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 32,997 $ 18,481 Research and development tax credit carryforwards 25,405 14,991 Capitalized organization costs 306 483 Stock-based compensation expense 7,714 5,624 Charitable Contributions 8 6 Deferred Revenue 26,480 42,742 Accrued expenses 4,613 5,560 Capitalized research and development expenses 73 115 Total deferred tax assets $ 97,596 88,002 Deferred tax liabilities: Depreciation and amortization (3,470 ) (5,008 ) Total deferred tax liabilities (3,470 ) (5,008 ) Valuation allowance $ (94,126 ) (82,994 ) Net deferred tax assets $ — $ — On December 22, 2017, the Tax Cuts and Jobs Act (the “TCJA”) was signed into United States law. The TCJA includes a number of changes to existing tax law, including, among other things, a permanent reduction in the federal corporate income tax rate from 35% to 21%, The staff of the Securities and Exchange Commission issued Staff Accounting Bulletin No. 118 to address the application of U.S. GAAP in situations when a registrant does not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the TCJA. In connection with the initial analysis of the impact of the TCJA, the Company remeasured its deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future, which is generally 21% for federal tax purposes. The remeasurement of the Company’s deferred tax assets and liabilities was offset by a change in the valuation allowance. The Company is still in the process of analyzing the impact to the Company of the TCJA and its analysis is not yet complete. Where the Company has been able to make reasonable estimates of the effects related to the TCJA, the Company has recorded provisional amounts. The ultimate impact to the Company’s consolidated financial statements of the TCJA may differ from the provisional amounts. The provisional amount recorded related to the remeasurement of our deferred tax balance was a $29,546 expense that was offset by a valuation allowance. As of December 31, 2017, the Company had net operating loss carryforwards for federal and state income tax purposes of $120,707 and $121,016, respectively, which both begin to expire in 2035. As of December 31, 2017, the Company also had available research and development tax credit carryforwards for federal and state income tax purposes of $22,557 and $ 3,606, respectively, which begin to expire in 2031 and 2028, respectively. The federal research and development tax credits include an orphan drug credit carryforward of $14,475. Utilization of the net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50% over a three-year period. Since our formation, we have raised capital through the issuance of capital stock on several occasions. These financings, combined with the purchasing shareholders' subsequent disposition of those shares, may have resulted in a change of control as defined by Section 382 or could result in a change of control in the future upon subsequent disposition. We conducted an analysis under Section 382 to determine if historical changes in ownership through August 31, 2015 would limit or otherwise restrict our ability to utilize these NOL and R&D credit carryforwards. As a result of this analysis, we do not believe there are any significant limitations on our ability to utilize these carryforwards. However, future changes in ownership after August 31, 2015 could affect the limitation in future years. Any limitation may result in expiration of a portion of the NOL or R&D credit carryforwards before utilization. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2017 and 2016. Management reevaluates the positive and negative evidence at each reporting period. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2017, 2016 and 2015 related primarily to the increases in net operating loss carryforwards, research and development tax credit carryforwards, stock-based compensation and decrease of deferred rate due to tax reform were as follows: Year Ended December 31, 2017 2016 2015 Valuation allowance at beginning of year $ (82,994 ) $ (32,777 ) $ (10,522 ) Decreases recorded as benefit to income tax provision 29,546 — — Increases recorded to income tax provision (40,678 ) (50,217 ) (22,255 ) Valuation allowance as of end of year $ (94,126 ) $ (82,994 ) $ (32,777 ) As of December 31, 2017, 2016, and 2015, the Company had no unrecognized tax benefits. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense. The Company does not expect any significant change in its uncertain tax positions in the next 12 months. During the year ended December 31, 2016, the Company was awarded a tax incentive for job creation from the Massachusetts Life Sciences Center. The program was established in 2008 in order to incentivize life sciences companies to create new sustained jobs in Massachusetts. Jobs must be maintained for at least five years, during which time the grant proceeds can be recovered by the Massachusetts Department of Revenue ("DOR") if the Company does not meet and maintain its job creation commitments.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Selected Quarterly Financial Data (Unaudited)	13 . Selected Quarterly Financial Data (Unaudited) The following table contains quarterly financial information for 2017 and 2016. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total Collaboration revenue - related party (1) $ 3,015 $ 3,014 $ 23,015 $ 3,056 $ 32,100 Total operating expenses 28,905 31,430 30,329 32,831 123,495 Loss from operations (25,890 ) (28,416 ) (7,314 ) (29,775 ) (91,395 ) Net loss (25,474 ) (28,018 ) (6,935 ) (28,953 ) (89,380 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.63 ) $ (0.69 ) $ (0.17 ) $ (0.72 ) $ (2.21 ) 2016 First Quarter Second Quarter Third Quarter Fourth Quarter Total Collaboration revenue - related party ( 1) $ 2,710 $ 3,004 $ 13,015 $ 3,037 $ 21,766 Total operating expenses 22,626 31,144 32,110 28,725 114,605 Loss from operations (19,916 ) (28,140 ) (19,095 ) (25,688 ) (92,839 ) Net loss (19,704 ) (27,913 ) (18,688 ) (25,274 ) (91,579 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.50 ) $ (0.70 ) $ (0.46 ) $ (0.63 ) $ (2.30 ) (1) In January 2016, the Company entered into a Collaboration and License Agreement with NHS for the development and commercialization of certain of its product candidates. In exchange for the license, NHS paid the Company an upfront cash payment of $120,000, which the Company received in February 2016 and is being recognized as revenue over the ten year performance period. During the quarter ended September 30, 2016, the Company received $10,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016. During the quarter ended September 30, 2017, the Company received $20,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2017.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	14 . Related Party Transactions In October 2010, the Company entered into a services agreement with Flagship Ventures Management, Inc., an affiliate of one of its stockholders (and now known as Flagship Pioneering), Flagship Venture Funds, to provide general and administrative services to the Company, including the employer portions of employee health and dental benefit plans for Seres Therapeutics employees and consulting services. The Company made payments under the agreement of $17 and $502 during the years ended December 31, 2016 and 2015, respectively. There were no payments made during the year ended December 31, 2017. There were no amounts due to Flagship Pioneering related to the services agreement as of December 31, 2017 and 2016. As described in Note 9, in January 2016 the Company entered into a License Agreement with NHS 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. NHS is a related party since NHS is an affiliate of Nestlé Health Science, one of the Company’s significant stockholders. During the year ended December 31, 2017, the Company recognized $32,100 of related party revenue associated with the License Agreement. As of December 31, 2017, there was $96,926 of deferred revenue related to the License Agreement, which is classified as current or non-current in the consolidated balance sheets. The Company has made no payments to NHS during the year ended December 31, 2017. There is $510 due from NHS as of December 31, 2017 for the reimbursement of development costs, which is classified as other current assets in the Company’s consolidated balance sheet.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
401(k) Savings Plan	15 . 401(k) Savings Plan The Company has a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Effective January 1, 2016, the Company has elected to match 50% of the first 6% of an employee’s deferral. Company contributions are expensed in the year for which they are declared. During the year ended December 31, 2017, the Company recorded expense of $484 for 401(k) match contributions.

Summary of Significant Accoun23

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
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 consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Cash Equivalents	Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market accounts, corporate bonds, and certificates of deposit purchased with original maturities of less than 90 days from the date of purchase, are stated at fair value.
Restricted Cash	Restricted Cash The Company held cash of $1,513 as of December 31, 2017 and $1,400 as of December 31, 2016 in a separate restricted bank account as a security deposit for the lease of the Company’s facilities. The Company has classified these deposits as long-term restricted cash on its balance sheet.
Investments	Investments The Company classifies its available-for-sale investments as current assets on the consolidated balance sheet if they mature within one year from the balance sheet date. The Company classifies all of its investments as available-for-sale securities. The Company’s investments are measured and reported at fair value using quoted prices in active markets for similar securities. Unrealized gains and losses on available-for-sale securities are reported as a separate component of stockholders’ equity (deficit). The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the consolidated statement of operations and comprehensive loss. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary”, the Company reduces the investment to fair value through a charge to the consolidated statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
Concentration of Credit Risk and of Significant Suppliers	Concentration of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. The Company has all cash, cash equivalents and investments balances at accredited financial institutions, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including pre-clinical and clinical testing. These programs could be adversely affected by a significant interruption in the supply of such drug substance products.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above. The Company’s investments in certificates of deposit are carried at amortized cost, which approximates fair value. Certain cash equivalents or investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The carrying values of the Company’s accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs (non-current) until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the financing.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset. Laboratory equipment is depreciated over five years. Computer equipment and furniture and office equipment are depreciated over three years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses include salaries, stock-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both pre-clinical studies and clinical trials.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Accounting for Stock-Based Compensation	Accounting for Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options, restricted stock units and restricted stock awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. For stock options or restricted stock units issued with performance-based vesting conditions, the stock compensation expense related to these awards is recognized based on the grant date fair value when achievement of the performance condition is deemed probable. The Company measures stock-based awards granted to consultants and non-employees based on the fair value of the award on the date on which the related service is complete. Compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. The Company recognizes adjustments to stock compensation expense for forfeitures as they occur. The fair value of each stock option grant is estimated on the date of grant using the Black- Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.
Revenue Recognition	Revenue Recognition The Company currently generates its revenue through collaboration and license arrangements with strategic partners for the development and commercialization of product candidates. The Company recognizes revenue in accordance with FASB ASC Topic 605, Revenue Recognition (“ASC 605”). Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met: • Persuasive evidence of an arrangement exists • Delivery has occurred or services have been rendered • The seller’s price to the buyer is fixed or determinable • Collectability is reasonably assured Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as long-term deferred revenue.
Collaboration Revenue	Collaboration Revenue In January 2016 the Company entered into a Collaboration and License Agreement (the “License Agreement”) with Nestec Ltd. (“NHS”), an affiliate of Nestlé Health Science US Holdings, Inc. In connection with the License Agreement, the Company received an upfront, non-refundable payment of $ 120,000 The Company evaluates multiple-element arrangements based on the guidance in FASB ASC Topic 605-25, Revenue Recognition-Multiple-Element Arrangements (“ASC 605-25”). Pursuant to this guidance, the Company identifies the deliverables included in the arrangement and determines: (1) whether the individual deliverables have value to the customer on a standalone basis and represent separate units of accounting or whether they must be accounted for as a combined unit of accounting; and (2) if the arrangement includes a general right of return relative to the delivered item. This evaluation requires management to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that: (i) the delivered item(s) has value to the customer on a standalone basis and (ii) if the arrangement includes a general right of return relative to the delivered item(s), delivery or performance of the undelivered item(s) is considered probable and substantially in the control of the Company. In assessing whether an item has standalone value, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner, the retention of any key rights by the Company, and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use the other deliverable(s) for their intended purpose without the receipt of the remaining element(s), whether the value of the deliverable is dependent on the undelivered item(s) and whether there are other vendors that can provide the undelivered element(s). In situations where the Company has identified multiple units of accounting, the arrangement consideration that is fixed or determinable is allocated among the separate units of accounting using the relative selling price method. The Company determines the selling price of a unit of accounting following the hierarchy of evidence prescribed by ASC 605-25. Accordingly, the Company determines the estimated selling price for units of accounting within each arrangement using vendor-specific objective evidence (“VSOE”) of selling price, if available, third-party evidence (“TPE”) of selling price if VSOE is not available, or best estimate of selling price (“BESP”) if neither VSOE nor TPE is available. Then, the applicable revenue recognition criteria in ASC 605-25 are applied to each of the separate units of accounting to determine the appropriate period and pattern of recognition. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605-25 are satisfied for that particular unit of accounting. The Company will recognize as revenue, upon delivery, arrangement consideration attributed to licenses that have standalone value from the other deliverables to be provided in an arrangement. For licenses that do not have standalone value from the other deliverables to be provided in an arrangement over the Company’s estimated performance period as the arrangement would be accounted for as a single unit of accounting. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605-25 are satisfied for that particular unit of accounting. The Company will recognize as revenue arrangement consideration attributed to licenses that have standalone value from the other deliverables to be provided in an arrangement upon delivery. The Company will recognize as revenue arrangement consideration attributed to licenses that do not have standalone value from the other deliverables to be provided in an arrangement over the Company’s estimated performance period as the arrangement would be accounted for as a single unit of accounting. If there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement for the single unit of accounting on a straight-line basis over the period the Company is expected to complete its performance obligations. Alternatively, if the pattern of performance in which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight-line method or proportional performance method, as applicable. At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the respective milestone and the level of effort and investment required to achieve the respective milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all of the criteria required to conclude that a milestone is substantive. The Company recognizes revenue associated with substantive milestones in accordance with FASB ASC Topic 605-28, Revenue Recognition-Milestone Method upon successful accomplishment of each milestone, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive would be recognized as revenue over the remaining period of performance, assuming all other revenue recognition criteria are met. The Company will recognize royalty revenue in the period of sale of the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met. Refer to footnote 9 for further information related to the Company’s collaboration and license agreement with Nestec, Ltd.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
Warrant to Purchase Convertible Preferred Stock	Warrant to Purchase Convertible Preferred Stock The Company classified a warrant to purchase shares of its Series A-2 convertible preferred stock as a liability on its consolidated balance sheets as this warrant was a free-standing financial instrument that could have required the Company to transfer assets upon exercise. The warrant was initially recorded at fair value on date of grant, and it was subsequently remeasured to fair value at each balance sheet date. Changes in fair value of the warrant were recognized as a component of other income (expense), net in the consolidated statement of operations and comprehensive loss. The Company continued to adjust the liability for changes in fair value until the warrant became a warrant to purchase common stock in connection with the IPO. The Company used the Black-Scholes option-pricing model, which incorporates assumptions and estimates, to value the preferred stock warrant. The Company assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. Estimates and assumptions impacting the fair value measurement included the fair value per share of the underlying Series A-2 convertible preferred stock, the remaining contractual term of the warrant, the risk-free interest rate, the expected dividend yield and the expected volatility of the price of the underlying preferred stock. The Company determined the fair value per share of the underlying preferred stock by taking into consideration the most recent sales of its convertible preferred stock, results obtained from third-party valuations and additional factors that were deemed relevant. The Company historically has been a private company and lacks company-specific historical and implied volatility information of its stock. Therefore, it estimated its expected stock volatility based on the historical volatility of publicly traded peer companies for a term equal to the remaining contractual term of the warrant. The risk-free interest rate was determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrant. Expected dividend yield was based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.
Segment Data	Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing Ecobiotic microbiome therapeutics to treat dysbiosis in the colonic microbiome. Revenue to date has been generated solely through the Company's collaboration with Nestle Health Science, all of which has been earned in the United States. All tangible assets are held in the United States
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2017, 2016 and 2015, other comprehensive income (loss) consisted of changes in unrealized gains (losses) from available-for-sale investments.
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 warrants and unvested restricted stock. The Company applies the two-class method to calculate its basic and diluted net loss per share attributable to common stockholders, as its convertible preferred stock, which converted to Common Stock upon completion of the listing of the Company’s common stock on NASDAQ on June 26, 2015, and common stock are participating securities. The two-class method is an earnings allocation formula that treats a participating security as having rights to earnings that otherwise would have been available to common stockholders. However, the two-class method does not impact the net loss per share of common stock as the Company was in a net loss position for each of the periods presented and preferred stockholders do not participate in losses. The Company’s convertible preferred stock contractually entitles the holders of such shares to participate in dividends but does not contractually require the holders of such shares to participate in losses of the Company. Similarly, restricted stock awards granted by the Company entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding common shares at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers The Company adopted ASC 606 using the modified retrospective transition method. The adoption of ASC 606 changed the pattern and timing of revenue recognition of amounts from the Company’s collaboration agreement with NHS. Under ASC 606, the Company will recognize revenue using the cost-to-cost method which best depicts the transfer of control to the customer. Under the cost-to-cost method, the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the identified performance obligation. Under this method, revenue will be recorded as a percentage of the estimated transaction price based on the extent of progress towards completion. The estimate of the Company’s measure of progress and estimate of transaction price will be updated at each reporting date, as a change in estimate, and will require judgement. The amount of consideration allocated to satisfied performance obligations, based on the Company’s measure of progress will be recognized immediately on a cumulative catch-up basis, resulting in an adjustment to revenue in the period of change. The amount related to the unsatisfied portion will be recognized as that portion is satisfied over time. In addition, substantive development milestones, which were previously recognized in the period when the milestone was achieved, will be recognized over the remaining performance period under ASC 606. As the adoption method does not result in a recast of the prior year consolidated financial statements, ASC 606 requires the Company to provide additional disclosures during the year of adoption of the amount by which each financial statement line item is affected by adoption of the new standard and explanations of the reasons for significant changes. During the fourth quarter of 2017, the Company substantially completed its impact assessment, and expects that the adoption of ASC 606 will require a cumulative-effect adjustment of approximately $25,000 In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842) In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU 2016-18, Restricted Cash In May 2017, the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting

Fair Value of Financial Asset24

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s assets and liabilities as of December 31, 2017 and 2016 that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of December 31, 2017 Using: Level 1 Level 2 Level 3 Not Leveling (1) Total Assets: Cash Equivalents $ — $ — $ — $ 25,964 $ 25,964 Investments: Commercial Paper — 6,198 — — 6,198 Certificates of Deposit — 8,916 — — 8,916 Corporate Bonds — 58,865 — — 58,865 Government Securities — 22,954 — — 22,954 Treasury Bonds — 16,962 — — 16,962 $ — $ 113,895 $ — $ 25,964 $ 139,859 (1) Fair Value Measurements as of December 31, 2016 Using: Level 1 Level 2 Level 3 Not Subject to Leveling (1) Total Assets: Cash Equivalents $ — $ 4,740 $ — $ 1,567 $ 6,307 Repurchase Agreements — 7,000 — — 7,000 Investments: Commercial Paper — 19,689 — — 19,689 Certificates of Deposit — 10,629 — — 10,629 Corporate Bonds — 94,609 — — 94,609 Government Securities — 33,466 — — 33,466 Treasury Bonds — 17,063 — — 17,063 $ — $ 187,196 $ — $ 1,567 $ 188,763 (1)

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Schedule of Fair Value of Available-for-Sale Investments by Type of Security	As of December 31, 2017 and 2016, the fair value of available-for-sale investments by type of security was as follows: December 31, 2017 Amortized Cost Gross Unrealized Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 6,198 $ — $ — $ 6,198 Certificates of Deposit 8,916 — — 8,916 Corporate Bonds 58,937 — (72 ) 58,865 Government Securities 22,997 — (43 ) 22,954 Treasury Bonds 16,992 — (30 ) 16,962 $ 114,040 $ — $ (145 ) $ 113,895 December 31, 2016 Amortized Cost Gross Unrealized Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 19,631 $ 58 $ — $ 19,689 Certificates of Deposit 10,629 — — 10,629 Corporate Bonds 94,764 — (155 ) 94,609 Government Securities 33,513 — (47 ) 33,466 Treasury Bonds 17,066 1 (4 ) 17,063 $ 175,603 $ 59 $ (206 ) $ 175,456

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following: December 31, 2017 December 31, 2016 Laboratory equipment $ 13,181 $ 10,711 Computer equipment 2,832 1,335 Furniture and office equipment 1,033 1,010 Leasehold improvements 27,963 27,807 Construction in progress 361 442 45,370 41,305 Less: Accumulated depreciation and amortization (12,439 ) (5,180 ) $ 32,931 $ 36,125

Accrued Expenses and Other Cu27

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: December 31, 2017 December 31, 2016 Development and clinical manufacturing costs $ 3,910 $ 3,350 Payroll and payroll-related costs 4,962 3,698 Professional fees 344 448 Facility and other 3,297 3,316 $ 12,513 $ 10,812

Stockholders_ Equity Common S28

Stockholders’ Equity Common Stock (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Summary of Fair Value of Stock Option	The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis: Year Ended December 31, 2017 2016 2015 Risk-free interest rate 2.20 % 1.54 % 1.80 % Expected term (in years) 6.0 6.0 6.0 Expected volatility 80.9 % 84.2 % 81.4 % Expected dividend yield 0 % 0 % 0 %
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2017: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2016 5,069,133 $ 14.36 8.26 $ 16,736 Granted 1,749,000 10.21 Exercised (174,386 ) 0.67 Forfeited (518,055 ) 21.07 Outstanding as of December 31, 2017 6,125,692 $ 13.00 7.70 $ 15,808 Options vested and expected to vest as of December 31, 2017 6,125,692 $ 13.00 7.70 $ 15,808 Options exercisable as of December 31, 2017 3,235,166 $ 11.37 7.00 $ 14,074
Summary of Restricted Stock Activity	The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2017: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted stock units as of December 31, 2016 115,500 $ 9.78 Granted 332,500 $ 10.08 Forfeited (47,100 ) $ 9.93 Vested (44,122 ) $ 10.02 Unvested restricted stock units as of December 31, 2017 356,778 $ 10.01
Summary of Stock Based Compensation Expense	The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its consolidated statements of operations and comprehensive loss: Year Ended December 31, 2017 2016 2015 Research and development expenses $ 8,115 $ 8,310 $ 5,297 General and administrative expenses 9,247 8,547 4,397 $ 17,362 $ 16,857 $ 9,694

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, 2017 2016 2015 Numerator: Net loss attributable to common stockholders $ (89,380 ) $ (91,579 ) $ (54,780 ) Denominator: Weighted average common shares outstanding, basic and diluted 40,449,410 39,846,928 23,532,400 Net loss per share attributable to common stockholders, basic and diluted $ (2.21 ) $ (2.30 ) $ (2.33 )
Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share	The following potential common shares, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2017 2016 2015 Stock options to purchase common stock 6,125,692 5,069,133 5,026,246 Unvested restricted stock units 356,778 115,500 — 6,482,470 5,184,633 5,026,246

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments for Operating Leases	Future minimum lease payments for these operating leases as of December 31, 2017 are as follows: Year Ending December 31, 2018 $ 6,157 2019 6,342 2020 6,120 2021 6,221 2022 6,372 2023 and thereafter 5,158 Total $ 36,370

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2017 2016 2015 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% Research and development tax credits (7.7 ) (8.8 ) (5.7 ) State taxes, net of federal benefit (4.8 ) (5.8 ) (5.3 ) Stock-based compensation 1.2 (6.3 ) 4.1 Revaluation of preferred stock warrant liability 33.1 — — Other (0.3 ) 0.1 0.3 Change in deferred tax asset valuation allowance 12.5 54.8 40.6 Effective income tax rate — % — % — %
Schedule of Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2017 and 2016 consisted of the following: December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 32,997 $ 18,481 Research and development tax credit carryforwards 25,405 14,991 Capitalized organization costs 306 483 Stock-based compensation expense 7,714 5,624 Charitable Contributions 8 6 Deferred Revenue 26,480 42,742 Accrued expenses 4,613 5,560 Capitalized research and development expenses 73 115 Total deferred tax assets $ 97,596 88,002 Deferred tax liabilities: Depreciation and amortization (3,470 ) (5,008 ) Total deferred tax liabilities (3,470 ) (5,008 ) Valuation allowance $ (94,126 ) (82,994 ) Net deferred tax assets $ — $ —
Summary of Changes In Valuation Allowance for Deferred Tax Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2017, 2016 and 2015 related primarily to the increases in net operating loss carryforwards, research and development tax credit carryforwards, stock-based compensation and decrease of deferred rate due to tax reform were as follows: Year Ended December 31, 2017 2016 2015 Valuation allowance at beginning of year $ (82,994 ) $ (32,777 ) $ (10,522 ) Decreases recorded as benefit to income tax provision 29,546 — — Increases recorded to income tax provision (40,678 ) (50,217 ) (22,255 ) Valuation allowance as of end of year $ (94,126 ) $ (82,994 ) $ (32,777 )

Selected Quarterly Financial 32

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Schedule of Quarterly Financial Information	The following table contains quarterly financial information for 2017 and 2016. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total Collaboration revenue - related party (1) $ 3,015 $ 3,014 $ 23,015 $ 3,056 $ 32,100 Total operating expenses 28,905 31,430 30,329 32,831 123,495 Loss from operations (25,890 ) (28,416 ) (7,314 ) (29,775 ) (91,395 ) Net loss (25,474 ) (28,018 ) (6,935 ) (28,953 ) (89,380 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.63 ) $ (0.69 ) $ (0.17 ) $ (0.72 ) $ (2.21 ) 2016 First Quarter Second Quarter Third Quarter Fourth Quarter Total Collaboration revenue - related party ( 1) $ 2,710 $ 3,004 $ 13,015 $ 3,037 $ 21,766 Total operating expenses 22,626 31,144 32,110 28,725 114,605 Loss from operations (19,916 ) (28,140 ) (19,095 ) (25,688 ) (92,839 ) Net loss (19,704 ) (27,913 ) (18,688 ) (25,274 ) (91,579 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.50 ) $ (0.70 ) $ (0.46 ) $ (0.63 ) $ (2.30 ) (1) In January 2016, the Company entered into a Collaboration and License Agreement with NHS for the development and commercialization of certain of its product candidates. In exchange for the license, NHS paid the Company an upfront cash payment of $120,000, which the Company received in February 2016 and is being recognized as revenue over the ten year performance period. During the quarter ended September 30, 2016, the Company received $10,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016. During the quarter ended September 30, 2017, the Company received $20,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2017.

Nature of the Business and Ba33

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporated year									Oct. 31, 2010
Entity incorporated state name									State of Delaware
Accumulated deficit	$ 263,571				$ 174,191				$ 263,571	$ 174,191
Net loss	(28,953)	$ (6,935)	$ (28,018)	$ (25,474)	$ (25,274)	$ (18,688)	$ (27,913)	$ (19,704)	(89,380)	(91,579)	$ (54,780)
Cash used in operations									(75,523)	$ 43,921	$ (40,844)
Cash, cash equivalents and investments	$ 149,983								$ 149,983

Summary of Significant Accoun34

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Feb. 29, 2016	Jan. 31, 2016	Dec. 31, 2017	Jan. 01, 2018	Dec. 31, 2016
Accounting Policies [Line Items]
Maximum maturity days for cash equivalents			90 days
Restricted cash			$ 1,513,000		$ 1,400,000
Impairment losses on long lived assets			$ 0
Minimum percentage of likelihood of tax benefit being realized upon settlement			50.00%
ASC 606 [Member] | Subsequent Event [Member]
Accounting Policies [Line Items]
Cumulative effect of accounting changes made upon adoption of new accounting principle				$ 25,000,000
Nestlé Health Science [Member]
Accounting Policies [Line Items]
Upfront non-refundable payment	$ 120,000,000	$ 120,000,000	$ 120,000,000
Laboratory Equipment [Member]
Accounting Policies [Line Items]
Useful life of asset			5 years
Computer Equipment [Member]
Accounting Policies [Line Items]
Useful life of asset			3 years
Furniture and Office Equipment [Member]
Accounting Policies [Line Items]
Useful life of asset			3 years

Fair Value of Financial Asset35

Fair Value of Financial Assets and Liabilities - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Assets:
Cash Equivalents		$ 25,964	$ 6,307
Repurchase Agreements			7,000
Investments:
Total assets		139,859	188,763
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash Equivalents			4,740
Repurchase Agreements			7,000
Investments:
Total assets		113,895	187,196
Not Subject to Leveling [Member]
Assets:
Cash Equivalents	[1]	25,964	1,567
Investments:
Total assets	[1]	25,964	1,567
Commercial Paper [Member]
Investments:
Investments		6,198	19,689
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		6,198	19,689
Certificates of Deposit [Member]
Investments:
Investments		8,916	10,629
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		8,916	10,629
Corporate Bonds [Member]
Investments:
Investments		58,865	94,609
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		58,865	94,609
Government Securities [Member]
Investments:
Investments		22,954	33,466
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		22,954	33,466
Treasury Bonds [Member]
Investments:
Investments		16,962	17,063
Treasury Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		$ 16,962	$ 17,063
[1]	Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy.

Fair Value of Financial Asset36

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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
Fair Value, assets transfers into (out of) Level 3 measurement	$ 0	$ 0

Investments - Schedule of Fair

Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 114,040	$ 175,603
Gross Unrealized Gain		59
Gross Unrealized Loss	(145)	(206)
Fair Value	113,895	175,456
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,198	19,631
Gross Unrealized Gain		58
Fair Value	6,198	19,689
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,916	10,629
Fair Value	8,916	10,629
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58,937	94,764
Gross Unrealized Loss	(72)	(155)
Fair Value	58,865	94,609
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,997	33,513
Gross Unrealized Loss	(43)	(47)
Fair Value	22,954	33,466
Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,992	17,066
Gross Unrealized Gain		1
Gross Unrealized Loss	(30)	(4)
Fair Value	$ 16,962	$ 17,063

Investments - Additional Inform

Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017
Investment [Line Items]
Maximum maturity days for cash equivalents	90 days
Corporate Bonds, Commercial Paper, Government Securities and Treasury Bonds [Member]
Investment [Line Items]
Remaining maturity period of investments	12 months

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 45,370	$ 41,305
Less: Accumulated depreciation and amortization	(12,439)	(5,180)
Property and equipment, net	32,931	36,125
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	13,181	10,711
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,832	1,335
Furniture and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,033	1,010
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27,963	27,807
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 361	$ 442

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 7,259	$ 4,205	$ 728

Accrued Expenses and Other Cu41

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Development and clinical manufacturing costs	$ 3,910	$ 3,350
Payroll and payroll-related costs	4,962	3,698
Professional fees	344	448
Facility and other	3,297	3,316
Total Accrued Expenses and Other Current Liabilities	$ 12,513	$ 10,812

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - $ / shares	Dec. 31, 2017	Dec. 31, 2016	Jul. 01, 2015
Equity [Abstract]
Preferred stock, shares authorized	10,000,000	10,000,000	10,000,000
Preferred stock, par value	$ 0.001	$ 0.001	$ 0.001

Stockholders' Equity Common Sto

Stockholders' Equity Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jul. 01, 2015	Jun. 26, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized				200,000,000	200,000,000	38,000,000
Common stock, par value				$ 0.001	$ 0.001	$ 0.001
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock issued and sold	8,545,138		8,545,138
Net proceeds from issuance of common stock from IPO after deducting underwriting discounts and commissions	$ 139,267
Estimated stock offering expenses	$ 3,748
Conversion of redeemable convertible preferred stock into common stock		22,866,987	22,866,987
Initial Public Offering ("IPO") [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized	200,000,000
Common stock, par value	$ 0.001
Initial Public Offering ("IPO") [Member] | Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Public offering price per share	$ 18
Over-Allotment Option [Member] | Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock issued and sold	1,114,583

Stockholders' Equity Common S44

Stockholders' Equity Common Stock - 2012 Stock Incentive plan - Additional Information (Detail) - 2012 Stock Incentive Plan [Member]	12 Months Ended
	Dec. 31, 2017shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plan description	The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. The Company generally grants stock-based awards with service conditions only (“service-based” awards).
Shares available for future grant	0
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price per share of common stock expressed as a percentage	100.00%
Stock option granted vesting period	4 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option term	10 years
Stock option granted vesting period	4 years
Stock option expiration period	10 years

Stockholders' Equity Common S45

Stockholders' Equity Common Stock - 2015 Incentive Award Plan - Additional Information (Detail) - 2015 Incentive Award Plan [Member] - shares	Jun. 16, 2015	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares initially reserved for issuance	2,200,000
Percentage of common stock shares outstanding	4.00%
Stock incentive plan description		The number of shares initially reserved for issuance under the 2015 Plan is the sum of (i) 2,200,000 shares of common stock and (ii) the number of shares subject to awards outstanding under the 2012 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company on or after the effective date of the 2015 Plan. In addition, the number of shares of common stock that may be issued under the 2015 Plan is subject to increase on the first day of each calendar year, beginning in 2016 and ending in 2025, equal to the lesser of (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the preceding applicable calendar year and (ii) an amount determined by the Company’s board of directors.
Shares available for future grant		1,450,792

Stockholders' Equity Common S46

Stockholders' Equity Common Stock - 2015 Employee Stock Purchase Plan - Additional Information (Detail) - 2015 Employee Stock Purchase Plan [Member] - shares	Jun. 16, 2015	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate shares of common stock reserved for issuance	365,000
Shares issued under ESPP	400,000	0
Percentage of common stock shares outstanding	1.00%

Stockholders' Equity Common S47

Stockholders' Equity Common Stock - Summary of Estimated Fair Value of Employee Stock Options (Detail) - Stock Option Valuation [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.20%	1.54%	1.80%
Expected term (in years)	6 years	6 years	6 years
Expected volatility	80.90%	84.20%	81.40%
Expected dividend yield	0.00%	0.00%	0.00%

Stockholders' Equity Common S48

Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Number of shares, Beginning Balance	5,069,133
Number of Shares, Granted	1,749,000
Number of Shares, Exercised	(174,386)
Number of Shares, Forfeited	(518,055)
Number of shares, Ending Balance	6,125,692	5,069,133
Number of Shares, Options vested and expected to vest	6,125,692
Number of Shares, Options exercisable	3,235,166
Weighted Average Exercise Price, Beginning Balance	$ 14.36
Weighted Average Exercise Price, Granted	10.21
Weighted Average Exercise Price, Exercised	0.67
Weighted Average Exercise Price, Forfeited	21.07
Weighted Average Exercise Price, Ending Balance	13	$ 14.36
Weighted Average Exercise Price, Options vested and expected to vest	13
Weighted Average Exercise Price, Options exercisable	$ 11.37
Weighted Average Remaining Contractual Term, Outstanding	7 years 8 months 12 days	8 years 3 months 3 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest	7 years 8 months 12 days
Weighted Average Remaining Contractual Term, Options exercisable	7 years
Aggregate Intrinsic Value, Outstanding	$ 15,808	$ 16,736
Aggregate Intrinsic Value, Options vested and expected to vest	15,808
Aggregate Intrinsic Value, Options exercisable	$ 14,074

Stockholders' Equity Common S49

Stockholders' Equity Common Stock - Stock Options - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options	$ 7.07	$ 18.45	$ 14.56
Intrinsic Value, Stock options exercised	$ 1,912	$ 34,871	$ 4,125
Stock based compensation expense for stock options	$ 17,362	$ 16,857	$ 9,694
Outstanding unvested service-based stock options	45,000	63,375
Stock Option [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-Based stock options vested		60,000
Stock based compensation expense for stock options		$ 235
Grant date fair value of options		$ 3.92

Stockholders' Equity Common S50

Stockholders' Equity Common Stock - Summary of Restricted Stock Activity (Detail) - Restricted Stock Units [Member]	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested restricted stock units, Beginning balance | shares	115,500
Number of Shares, Granted | shares	332,500
Number of Shares, Forfeited | shares	(47,100)
Number of Shares, Vested | shares	(44,122)
Number of Shares, Unvested restricted stock units, Ending balance | shares	356,778
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Beginning balance | $ / shares	$ 9.78
Weighted Average Grant Date Fair Value, Granted | $ / shares	10.08
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	9.93
Weighted Average Grant Date Fair Value, Vested | $ / shares	10.02
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Ending balance | $ / shares	$ 10.01

Stockholders' Equity Common S51

Stockholders' Equity Common Stock - Restricted Stock Units - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value, vested	$ 445	$ 0	$ 1,508

Stockholders' Equity Common S52

Stockholders' Equity Common Stock - Summary of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 17,362	$ 16,857	$ 9,694
Research and development expenses [Member] | Stock Options and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	8,115	8,310	5,297
General and administrative expenses [Member] | Stock Options and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 9,247	$ 8,547	$ 4,397

Stockholders' Equity Common S53

Stockholders' Equity Common Stock - Stock Based Compensation - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Equity [Abstract]
Unrecognized stock- based compensation cost	$ 28,102
Weighted average period over which unrecognized compensation cost is expected to be recognized	2 years 1 month 20 days

Collaboration Revenue - Additio

Collaboration Revenue - Additional Information (Detail) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended								12 Months Ended
		Feb. 29, 2016	Jan. 31, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Period of performance identifiable deliverables			10 years
Collaboration revenue - related party	[1]			$ 3,056	$ 23,015	$ 3,014	$ 3,015	$ 3,037	$ 13,015	$ 3,004	$ 2,710	$ 32,100	$ 21,766
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment		$ 120,000	$ 120,000									120,000
Development milestone payments			285,000
Regulatory payments			375,000
Sales based milestones			$ 1,125,000
Period of performance identifiable deliverables			10 years
Reimbursement of development services												793
Proceeds on achievement of development milestone					$ 20,000				$ 10,000			20,000	10,000
Collaboration revenue associated with substantive milestones - related party												20,000	10,000
Collaboration revenue - related party												32,100	$ 21,766
Deferred revenue				$ 96,926								$ 96,926
[1]	In January 2016, the Company entered into a Collaboration and License Agreement with NHS for the development and commercialization of certain of its product candidates. In exchange for the license, NHS paid the Company an upfront cash payment of $120,000, which the Company received in February 2016 and is being recognized as revenue over the ten year performance period. During the quarter ended September 30, 2016, the Company received $10,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016. During the quarter ended September 30, 2017, the Company received $20,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2017.

Net Loss per Share - Basic and

Net Loss per Share - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss attributable to common stockholders									$ (89,380)	$ (91,579)	$ (54,780)
Denominator:
Weighted average common shares outstanding, basic and diluted									40,449,410	39,846,928	23,532,400
Net loss per share attributable to common stockholders, basic and diluted	$ (0.72)	$ (0.17)	$ (0.69)	$ (0.63)	$ (0.63)	$ (0.46)	$ (0.70)	$ (0.50)	$ (2.21)	$ (2.30)	$ (2.33)

Net Loss per Share - Summary of

Net Loss per Share - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	6,482,470	5,184,633	5,026,246
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	6,125,692	5,069,133	5,026,246
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	356,778	115,500

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Nov. 11, 2015USD ($)ft²	Apr. 02, 2015	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Sep. 30, 2016USD ($)
Other Commitments [Line Items]
Operating lease agreement expiration date			Jun. 30, 2016
Operating lease termination write off of deferred rent benefit			$ 136,000
Outstanding security deposit						$ 119,000
Lease incentive obligation			8,989,000	$ 10,730,000
Indemnification Agreement [Member]
Other Commitments [Line Items]
Indemnification obligations accrued			$ 0	0
BMR Sidney Research Campus LLC [Member]
Other Commitments [Line Items]
Non-cancelable property lease, description			On November 11, 2015, the Company entered into a non-cancelable property lease with BMR-Sidney Research Campus LLC (“BMR”) for 83,396 square feet of office, laboratory and pilot manufacturing space at 200 Sidney Street, Cambridge, Massachusetts. The lease term commenced in March 2016 and ends in November 2023. The Company has the option to extend the lease twice, each for a five-year period. The Company moved its corporate headquarters to this location in April 2016
Square feet of property subject to non-cancelable lease | ft²	83,396
Lease commencement period			2016-03
Lease expiration period			2023-11
Cost of tenant improvements	$ 12,509,000
Lease incentive obligation			$ 10,312,000
Amortized of lease incentive obligation			1,768,000
Office and Laboratory Space [Member]
Other Commitments [Line Items]
Operating lease agreement expiration date		Apr. 30, 2020
Rental expense related to office and laboratory space			$ 4,458,000	$ 3,826,000	$ 1,246,000

Commitments and Contingencies58

Commitments and Contingencies - Summary Of Future Minimum Lease Payments For Operating Leases (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,018	$ 6,157
2,019	6,342
2,020	6,120
2,021	6,221
2,022	6,372
2023 and thereafter	5,158
Total	$ 36,370

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Income tax benefit			$ 0	$ 0	$ 0
Statutory federal corporate income tax rate			34.00%	34.00%	34.00%
Percentage of limitation of deduction for net operating losses to taxable income			80.00%
Provisional amount recorded related to remeasurement of deferred tax expense	$ 0		$ 29,546,000
Unrecognized tax benefits			0	$ 0	$ 0
Massachusetts Life Sciences Center [Member]
Income Tax Disclosure [Line Items]
Tax incentives awarded			$ 300,000
Tax incentive program period				5 years
Scenario Plan [Member]
Income Tax Disclosure [Line Items]
Statutory federal corporate income tax rate		21.00%
Maximum [Member]
Income Tax Disclosure [Line Items]
Statutory federal corporate income tax rate			35.00%
Federal [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards			$ 120,707,000
Net operating loss carry forwards, expiration year, start			2,035
Research and development tax credit carryforwards, expiration year, start			2,031
Federal [Member] | Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Research and development tax credit carryforwards			$ 22,557,000
Federal [Member] | Orphan Drug Credit Carryforwards [Member]
Income Tax Disclosure [Line Items]
Research and development tax credit carryforwards			14,475,000
State [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards			$ 121,016,000
Net operating loss carry forwards, expiration year, start			2,035
Research and development tax credit carryforwards, expiration year, start			2,028
State [Member] | Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Research and development tax credit carryforwards			$ 3,606,000

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(34.00%)	(34.00%)	(34.00%)
Research and development tax credits	(7.70%)	(8.80%)	(5.70%)
State taxes, net of federal benefit	(4.80%)	(5.80%)	(5.30%)
Stock-based compensation	1.20%	(6.30%)	4.10%
Revaluation of preferred stock warrant liability	33.10%
Other	(0.30%)	0.10%	0.30%
Change in deferred tax asset valuation allowance	12.50%	54.80%	40.60%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating loss carryforwards	$ 32,997	$ 18,481
Research and development tax credit carryforwards	25,405	14,991
Capitalized organization costs	306	483
Stock-based compensation expense	7,714	5,624
Charitable Contributions	8	6
Deferred Revenue	26,480	42,742
Accrued expenses	4,613	5,560
Capitalized research and development expenses	73	115
Total deferred tax assets	97,596	88,002
Deferred tax liabilities:
Depreciation and amortization	(3,470)	(5,008)
Total deferred tax liabilities	(3,470)	(5,008)
Valuation allowance	$ (94,126)	$ (82,994)	$ (32,777)	$ (10,522)

Income Taxes - Summary of Chang

Income Taxes - Summary of Changes In Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Abstract]
Valuation allowance at beginning of year	$ (82,994)	$ (32,777)	$ (10,522)
Decreases recorded as benefit to income tax provision	29,546
Increases recorded to income tax provision	(40,678)	(50,217)	(22,255)
Valuation allowance as of end of year	$ (94,126)	$ (82,994)	$ (32,777)

Selected Quarterly Financial 63

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Collaboration revenue - related party	[1]	$ 3,056	$ 23,015	$ 3,014	$ 3,015	$ 3,037	$ 13,015	$ 3,004	$ 2,710	$ 32,100	$ 21,766
Total operating expenses		32,831	30,329	31,430	28,905	28,725	32,110	31,144	22,626	123,495	114,605	$ 54,856
Loss from operations		(29,775)	(7,314)	(28,416)	(25,890)	(25,688)	(19,095)	(28,140)	(19,916)	(91,395)	(92,839)	(54,856)
Net loss		$ (28,953)	$ (6,935)	$ (28,018)	$ (25,474)	$ (25,274)	$ (18,688)	$ (27,913)	$ (19,704)	$ (89,380)	$ (91,579)	$ (54,780)
Net loss per share attributable to common stockholders, basic and diluted		$ (0.72)	$ (0.17)	$ (0.69)	$ (0.63)	$ (0.63)	$ (0.46)	$ (0.70)	$ (0.50)	$ (2.21)	$ (2.30)	$ (2.33)
[1]	In January 2016, the Company entered into a Collaboration and License Agreement with NHS for the development and commercialization of certain of its product candidates. In exchange for the license, NHS paid the Company an upfront cash payment of $120,000, which the Company received in February 2016 and is being recognized as revenue over the ten year performance period. During the quarter ended September 30, 2016, the Company received $10,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016. During the quarter ended September 30, 2017, the Company received $20,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2017.

Selected Quarterly Financial 64

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Parenthetical) (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		12 Months Ended
	Feb. 29, 2016	Jan. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information [Line Items]
Performance period for revenue recognized		10 years
Nestlé Health Science [Member]
Quarterly Financial Information [Line Items]
Upfront cash payment	$ 120,000	$ 120,000			$ 120,000
Proceeds on achievement of development milestone			$ 20,000	$ 10,000	20,000	$ 10,000
Collaboration revenue recognized for milestone - related party					$ 20,000	$ 10,000
Performance period for revenue recognized		10 years

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Collaboration revenue - related party	[1]	$ 3,056,000	$ 23,015,000	$ 3,014,000	$ 3,015,000	$ 3,037,000	$ 13,015,000	$ 3,004,000	$ 2,710,000	$ 32,100,000	$ 21,766,000
Flagship Pioneering [Member] | Services Agreement [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party										0	17,000	$ 502,000
Due to related party		0				$ 0				0	0
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party										0
Collaboration revenue - related party										32,100,000	$ 21,766,000
Deferred revenue		96,926,000								96,926,000
Due from related party for the reimbursement of development costs		$ 510,000								$ 510,000
[1]	In January 2016, the Company entered into a Collaboration and License Agreement with NHS for the development and commercialization of certain of its product candidates. In exchange for the license, NHS paid the Company an upfront cash payment of $120,000, which the Company received in February 2016 and is being recognized as revenue over the ten year performance period. During the quarter ended September 30, 2016, the Company received $10,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016. During the quarter ended September 30, 2017, the Company received $20,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2017.

401(k) Savings Plan - Additiona

401(k) Savings Plan - Additional Information (Detail) - USD ($) $ in Thousands	Jan. 01, 2016	Dec. 31, 2017
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Employer match of employee contributions	50.00%
Employee deferral subject to employer match	6.00%
Employer contribution expenses		$ 484