Seres Therapeutics (MCRB) 10-K 14 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2015	Mar. 07, 2016	Jun. 30, 2015
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
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	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		39,187,017
Entity Public Float			$ 422,395,134

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 73,933	$ 114,185
Investments	131,149
Prepaid expenses and other current assets	2,528	58
Total current assets	207,610	114,243
Property and equipment, net	7,751	1,264
Restricted cash	1,539	139
Deferred offering costs		1,684
Deferred financing costs		15
Total assets	216,900	117,345
Current liabilities:
Accounts payable	5,397	2,166
Accrued expenses and other current liabilities	5,523	1,737
Notes payable; current portion		1,200
Total current liabilities	10,920	5,103
Lease incentive obligation	586
Notes payable, net of discount		1,304
Preferred stock warrant liability		1,582
Total liabilities	$ 11,506	$ 7,989
Commitments and contingencies:
Stockholders’ equity (deficit):
Common stock, $0.001 par value; 200,000,000 and 38,000,000 shares authorized at December 31, 2015 and 2014, respectively; 39,082,017 and 6,890,250 shares issued and outstanding at December 31, 2015 and 2014, respectively	$ 39	$ 7
Additional paid-in capital	287,937	1,104
Accumulated other comprehensive income (loss)	30
Accumulated deficit	(82,612)	(27,832)
Total stockholders’ equity (deficit)	205,394	(26,721)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$ 216,900	117,345
Convertible Preferred Stock Series A, A-2, B, C, D and D-1 [Member]
Current liabilities:
Convertible preferred stock (Series A, A-2, B, C, D and D-1), value		$ 136,077

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	38,000,000
Common stock, shares issued	39,082,017	6,890,250
Common stock, shares outstanding	39,082,017	6,890,250
Convertible Preferred Stock Series A, A-2, B, C, D and D-1 [Member]
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	10,000,000	24,348,003
Convertible preferred stock, shares issued	0	22,866,987
Convertible preferred stock, shares outstanding	0	22,866,987
Convertible preferred stock, liquidation preference	$ 0	$ 137,283

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 0
Operating expenses:
Research and development expenses	38,095	$ 10,718	$ 4,805
General and administrative expenses	16,761	4,364	1,247
Total operating expenses	54,856	15,082	6,052
Loss from operations	(54,856)	(15,082)	(6,052)
Other income (expense):
Interest income	638
Interest expense	(555)	(209)	(42)
Revaluation of preferred stock warrant liability	(7)	(1,418)	(8)
Total other income (expense), net	76	(1,627)	(50)
Net loss	(54,780)	(16,709)	(6,102)
Accretion of convertible preferred stock to redemption value		(1,291)	(875)
Net loss attributable to common stockholders	$ (54,780)	$ (18,000)	$ (6,977)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.33)	$ (2.67)	$ (1.09)
Weighted average common shares outstanding, basic and diluted	23,532,400	6,748,037	6,394,916
Other comprehensive income (loss):
Unrealized gain on investments, net of tax of $0	$ 30
Total other comprehensive income	30
Comprehensive loss	$ (54,750)	$ (18,000)	$ (6,977)

Consolidated Statements of Ope5

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Unrealized 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	Convertible Preferred Stock Series A, A-2, B, C, D and D-1 [Member]	Series B Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Series D and D-1 Convertible Preferred Stock [Member]	Series D Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]
Stockholders' (deficit), value, beginning balance at Dec. 31, 2012	$ (4,348)						$ 7			$ (4,355)
Stockholders' (deficit), shares, beginning balance at Dec. 31, 2012							7,590,000
Convertible preferred stock, value, beginning balance at Dec. 31, 2012		$ 10,708
Convertible preferred stock, shares, beginning balance at Dec. 31, 2012		10,478,189
Repurchase of unvested restricted common stock							(735,000)
Stock-based compensation expense	209							$ 209
Accretion of convertible preferred stock to redemption value	(875)	$ 875								(666)
Accretion of convertible preferred stock to redemption value, impact on permanent equity								(209)
Net loss	(6,102)									(6,102)
Stockholders' (deficit), value, ending balance at Dec. 31, 2013	(11,116)						$ 7			(11,123)
Stockholders' (deficit), shares, ending balance at Dec. 31, 2013							6,855,000
Convertible preferred stock, value, ending balance at Dec. 31, 2013		$ 11,583
Convertible preferred stock, shares, ending balance at Dec. 31, 2013		10,478,189
Issuance of convertible preferred stock, net of issuance costs, value			$ 10,558	$ 47,813	$ 64,832
Issuance of convertible preferred stock, net of issuance costs, shares			4,831,359	3,946,328	3,611,111
Issuance of common stock upon exercise of stock options, value	5							5
Issuance of common stock upon exercise of stock options, shares							28,687
Issuance of common stock upon completion of initial public offering, net of offering costs, value	5							5
Issuance of common stock upon completion of initial public offering, net of offering costs, shares							6,563
Issuance of common stock upon exercise of common stock warrant, value	317							317
Stock-based compensation expense	2,068							2,068
Accretion of convertible preferred stock to redemption value	(1,291)	$ 1,291
Accretion of convertible preferred stock to redemption value, impact on permanent equity								(1,291)
Issuance costs of convertible preferred stock			$ (71)	$ (187)	$ (168)
Net loss	(16,709)									(16,709)
Stockholders' (deficit), value, ending balance at Dec. 31, 2014	$ (26,721)						$ 7	1,104		(27,832)
Stockholders' (deficit), shares, ending balance at Dec. 31, 2014	6,890,250						6,890,250
Convertible preferred stock, value, ending balance at Dec. 31, 2014		$ 136,077
Convertible preferred stock, shares, ending balance at Dec. 31, 2014		22,866,987
Issuance of common stock upon exercise of stock options, value	$ 93							93
Issuance of common stock upon exercise of stock options, shares	232,970						232,970
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
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
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	$ (136,053)					$ 23	136,030
Conversion of convertible preferred stock upon listing, shares		(22,866,987)					22,866,987
Unrealized gain 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						39,082,017
Convertible preferred stock, shares, ending balance at Dec. 31, 2015		0

Consolidated Statements of Con7

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2014USD ($)
Series B Convertible Preferred Stock [Member]
Issuance costs of convertible preferred stock	$ 71
Series C Convertible Preferred Stock [Member]
Issuance costs of convertible preferred stock	187
Series D and D-1 Convertible Preferred Stock [Member]
Issuance costs of convertible preferred stock	$ 168

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net loss	$ (54,780)	$ (16,709)	$ (6,102)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,694	2,068	209
Depreciation and amortization expense	728	190	88
Loss from revaluation of preferred stock warrant liability	7	1,418	8
Licensing fees paid in common stock warrant		317
Other non-cash expense	367	81	19
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,470)	(7)	(21)
Accounts payable	2,682	810	281
Accrued expenses and other liabilities	2,928	1,474	197
Net cash used in operating activities	(40,844)	(10,358)	(5,321)
Cash flows from investing activities:
Purchases of property and equipment	(4,362)	(1,001)	(174)
Purchases of Short-term investments	(267,761)
Maturities of Short-term investments	136,390
Changes in restricted cash	(1,400)	(102)	(10)
Net cash used in investing activities	(137,133)	(1,103)	(184)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	(24)	123,203
Proceeds from issuance of notes payable and preferred stock warrant, net of issuance costs		2,000	944
Proceeds from exercise of stock options and warrants	262	5
Proceeds from Issuance of common stock in connection with IPO	143,015
Proceeds from issuance of common stock and restricted common stock		5
Repayment of notes payable	(2,600)	(400)
Payments of initial public offering costs	(2,928)	(821)
Net cash provided by financing activities	137,725	123,992	944
Net increase (decrease) in cash and cash equivalents	(40,252)	112,531	(4,561)
Cash and cash equivalents at beginning of year	114,185	1,654	6,215
Cash and cash equivalents at end of year	73,933	114,185	1,654
Supplemental disclosure of cash flow information:
Cash paid for interest	163	122	20
Supplemental disclosure of non-cash investing and financing activities:
Accretion of convertible preferred stock to redemption value		1,291	875
Issuance of preferred stock warrant in connection with notes payable			$ 156
Deferred offering costs included in accounts payable		863
Property and equipment purchases included in accounts payable and accrued expenses	$ 2,953	$ 101

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2015
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 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’s consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. The Company has experienced negative cash flows and had an accumulated deficit of $82,612 and $27,832 as of December 31, 2015 and 2014, respectively. The Company expects that its cash, cash equivalents and investments at December 31, 2015 of $205,082 will enable it to fund its operating expense and capital expenditure requirements through at least December 31, 2016. The future viability of the Company is dependent on its ability to generate cash from operating activities or to raise additional capital to finance its operations. The Company’s failure to raise capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies. On July 1, 2015, the Company completed an initial public offering (“IPO”) of its common stock, and issued and sold 8,545,138 shares of common stock at a price to the public of $18.00 per share, resulting in net proceeds of approximately $139,267 after deducting underwriting discounts and commissions and offering expenses. 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 the NASDAQ Global Select Market (“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. In addition, at this time, the warrant to purchase shares of the Company’s Series A-2 convertible preferred stock was converted into a warrant to purchase shares of the Company’s common stock. 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, 2015
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, the accrual of research and development expenses and the valuation of common stock, stock-based awards and the preferred stock warrant liability. 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 commercial paper 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 $1,539 as of December 31, 2015 and $139 as of December 31, 2014 in a separate restricted bank account as a security deposit for the lease of the Company’s facilities and as collateral for the Company’s credit card program with Comerica Bank. 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 one accredited financial institution, 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, investments and preferred stock warrant liability are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s cash, prepaid expenses, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities. The carrying value of the Company’s outstanding debt as of December 31, 2014 approximates fair value based on the variable interest rate for the borrowings outstanding as well as short duration of the term of the note. The fair value of the outstanding debt was estimated using a discounted cash flow analysis based on current market interest rates for debt issuances with similar remaining years to maturity, adjusted for credit risk, which represents a Level 3 measurement. 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. As of December 31, 2015 and 2014 the Company had recorded $0 and $1,684, respectively, of deferred offering costs in contemplation of the IPO. On July 1, 2015, the Company reclassified $3,748 of deferred offering costs to additional paid in capital as a reduction of the proceeds from the IPO. 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, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options and restricted stock awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. 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 compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to stock- based compensation expense in future periods. 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. 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. No revenue has been generated since inception, and 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. There was no difference between net loss and comprehensive loss for the years ended December 31, 2014 and 2013. For the year ended December 31, 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 the 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 No. 2014-09, Revenue from Contracts with Customers Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, In April 2015, the FASB issued ASU 2015-03, Simplifying the Presentation of Debt Issuance Costs In May 2015, the FASB issued ASU No. 2015-07, Fair Value Measurement (Topic 820): Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent). The new standard removes the requirement to categorize within the fair value hierarchy all investments for which fair value is measured using the net asset value per share practical expedient. The new standard will be effective for us on January 1, 2016. Early application is permitted. The Company believes the adoption of this standard will not have a material impact on its financial statements. In November 2015, the FASB issued ASU 2015-17, “Income Taxes (Topic 740): Balance Sheet Classification of Deferred Taxes,” to simplify the presentation of deferred income taxes by requiring that deferred tax liabilities and assets be classified as noncurrent on the balance sheet. This update is effective for annual reporting periods beginning after December 31, 2016, including interim periods within those annual periods, and early adoption is permitted. Accordingly, the Company elected to early adopt ASU 2015-17 for the year ended December 31, 2015. There was no impact to the consolidated financial statements as a result of this change.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 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, 2015 Using: Level 1 Level 2 Level 3 Total Assets: Cash Equivalents $ — $ 23,125 $ — $ 23,125 Repurchase Agreements — 20,000 — 20,000 Investments: Commercial Paper — 64,820 — 64,820 Corporate Bonds — 46,490 — 46,490 Government Securities — 15,819 — 15,819 Treasury Bonds — 4,020 — 4,020 $ $ 174,274 $ $ 174,274 Fair Value Measurements as of December 31, 2014 Using: Level 1 Level 2 Level 3 Total Liabilities: Liability for preferred stock warrant $ — $ — $ 1,582 $ 1,582 $ — $ — $ 1,582 $ 1,582 As of December 31, 2015, the Company’s cash equivalents consisted of money market funds, corporate bonds, commercial paper, government securities 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. The fair value of the Company’s investments, which consisted of corporate bonds, commercial paper and government securities as of December 31, 2015, were determined using Level 2 inputs. During the years ended December 31, 2015 and 2014, there were no transfers between Level 1, Level 2 and Level 3. The liability for the preferred stock warrant in the amount of $1,582 in the table above as of December 31, 2014 is comprised of the value of a warrant for the purchase of Series A-2 convertible preferred stock and is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. There were no other assets or liabilities measured at a fair value on a recurring basis at December 31, 2015 or December 31, 2014. In connection with the automatic conversion of the Company’s convertible preferred stock, which occurred upon the listing of the Company’s common stock on the NASDAQ on June, 26, 2015, the preferred stock warrant become a warrant to purchase common stock and the liability was remeasured at fair value and reclassified to additional paid-in-capital. The fair value of the preferred stock warrant liability at the time was tied to the initial offering price of $18.00 per share of common stock.

Investments

Investments	12 Months Ended
	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
Investments	4. Investments As of December 31, 2015, the fair value of available-for-sale investments by type of security was as follows: December 31, 2015 Amortized Cost Gross Unrealized Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 64,733 $ 87 $ — $ 64,820 Corporate Bonds 46,538 — (48 ) 46,490 Government Securities 15,823 — (4 ) 15,819 Treasury Bonds 4,022 — (2 ) 4,020 $ 131,116 $ 87 $ (54 ) $ 131,149 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. The Company did not hold any investments as of December 31, 2014. As of December 31, 2015, the Company’s corporate bonds, commercial paper, government securities and treasury bonds had remaining maturities of less than 12 months.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following: December 31, 2015 December 31, 2014 Laboratory equipment $ 4,370 $ 1,260 Computer equipment 408 115 Furniture and office equipment 285 58 Leasehold improvements 1,856 114 Construction in progress 1,843 — 8,762 1,547 Less: Accumulated depreciation and amortization (1,011 ) (283 ) $ 7,751 $ 1,264 Depreciation and amortization expense was $728, $190 and $88 for the years ended December 31, 2015, 2014 and 2013, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2015
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, 2015 December 31, 2014 Development and clinical manufacturing costs $ 1,436 $ 598 Payroll and payroll-related costs 2,756 547 Professional fees 184 314 Facility and other 1,147 278 $ 5,523 $ 1,737

Notes Payable

Notes Payable	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
Notes Payable	7. Notes Payable On September 9, 2013, the Company entered into a loan and security agreement with Comerica Bank (“Comerica”), as amended on December 22, 2014, which provided for borrowings of up to $3,000 through August 2014. On September 9, 2013, the Company received $1,000 from borrowings under the agreement, and from March to August 2014, the Company received $2,000 from additional borrowings under the loan and security agreement. Through December 31, 2014, the Company borrowed the full $3,000 available under the loan and security agreement and had made $400 of scheduled principal repayments. During the year ended December 31, 2015, the Company made $900 of scheduled principal repayments. Borrowings under the loan and security agreement were collateralized by substantially all of the Company’s assets, except for its intellectual property. On September 17, 2015, the Company made a payment of $1,765 to Comerica to satisfy all amounts owed under the loan and security agreement. The extinguishment amount was comprised of $1,700 of outstanding principal and $65 of final fees and accrued interest. Upon payment, Comerica released the Company of all security interests held in the Company’s assets, except for the cash collateral securing the Company’s corporate cards and standby letters of credit, and terminated all loan documents related to the loan and security agreement (other than any indemnification obligations and other provisions which survive termination). In connection with the extinguishment of the loan and security agreement, the Company recorded a loss on extinguishment of $140, which has been recorded as interest expense in the year ended December 31, 2015. Accretion of the debt discount recorded as additional interest expense was $96 and $66 for the years ended December 31, 2015 and 2014, respectively. As of December 31, 2015 and December 31, 2014, the unamortized debt discount was $0 and $96, respectively.

Preferred Stock Warrant Liabili

Preferred Stock Warrant Liability	12 Months Ended
	Dec. 31, 2015
Class Of Stock Disclosures [Abstract]
Preferred Stock Warrant Liability	8. Preferred Stock Warrant Liability In September 2013, the Company issued a warrant to purchase 92,127 shares of Series A-2 convertible preferred stock in connection with the loan and security agreement (see Note 7). The warrant was immediately exercisable at an exercise price of $1.78 per share and had a contractual term of ten years from issuance. The fair value of the warrant at issuance was estimated to be $156 and was recorded as a debt discount and as a preferred stock warrant liability. The Company remeasured the fair value of the liability for this preferred stock warrant at each reporting date from its grant date, with any adjustments being recorded as a component of other income (expense), net in the Company’s consolidated statement of operations and comprehensive loss. The Company recorded losses of $7, $1,418 and $8 for the years ended December 31, 2015, 2014 and 2013, respectively, to reflect the change in fair value of this preferred stock warrant. The following assumptions and inputs were used in determining the fair value of the preferred stock warrant liability valued using the Black-Scholes option-pricing model: Year Ended December 31, 2015 2014 2013 Risk-free interest rate 2.40 % 2.17 % 3.20 % Expected term (in years) 8.2 8.7 9.7 Expected volatility 91.2 % 84.0 % 86.0 % Expected dividend yield 0 % 0 % 0 % Fair value of Series A-2 convertible preferred stock $ 17.26 $ 17.18 $ 2.07 The following table provides a rollforward of the fair value of the Company’s preferred stock warrant liability: Fair Value Balance as of December 31, 2012 $ — Issuance of Series A-2 preferred stock warrant 156 Loss on revaluation 8 Balance as of December 31, 2013 164 Loss on revaluation 1,418 Balance as of December 31, 2014 $ 1,582 Loss on revaluation 7 Reclassification to stockholders’ equity (1,589 ) Balance as of December 31, 2015 $ — In connection with the automatic conversion of the Company’s convertible preferred stock, which occurred upon the listing of the Company’s common stock on the NASDAQ on June 26, 2015, the preferred stock warrant became a warrant to purchase common stock. The Company performed the final mark to market adjustment on the preferred stock warrant using the fair value of the underlying common shares of $18.00 per share on June 26, 2015 and recorded the change in fair value in other income (expense), net in the consolidated statement of operations and comprehensive loss. The preferred stock warrant liability was then reclassified to additional paid-in-capital as it became a warrant to purchase common stock. On August 17, 2015, the common stock warrant was exercised in full at an exercise price of $1.78 per share of common stock. The Company received proceeds of $164 in connection with the exercise of the common stock warrant.

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Convertible Preferred Stock	9. 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. All outstanding shares of the Company’s convertible preferred stock automatically converted into 22,866,987 shares of the Company’s common stock upon the listing of the Company’s common stock on the NASDAQ on June 26, 2015.

Stockholders_ Equity (Deficit)

Stockholders’ Equity (Deficit) Common Stock	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Stockholders’ Equity (Deficit) Common Stock	10. Stockholders’ Equity (Deficit) 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 the 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. Common Stock Warrants In June 2014, the Company entered into a research agreement with the Mayo Foundation for Medical Education and Research (“Mayo”) under which the Company acquired a license to intellectual property. In exchange for the license, the Company issued to Mayo a warrant to purchase 454,545 shares of common stock at an exercise price of $0.01 per share, which was immediately exercisable. Upon issuance of the warrant, the Company recorded research and development expense of $317 for the fair value of the warrant, determined using the following assumptions in the Black-Scholes option-pricing model: expected volatility of 86.0%, risk-free interest rate of 2.3%, expected term of seven years (equaling the contractual term of the warrant) and no expected dividends. Because this warrant was indexed to the Company’s stock and could only be settled by gross physical delivery of shares the Company determined that this warrant qualified for equity classification. This warrant was exercised on April 29, 2015. The Company also issued to Mayo an additional warrant (the “performance warrant”) to purchase up to 284,090 shares of common stock at an exercise price equal to the per share price at which the Company most recently sold shares of its preferred stock. The performance warrant was exercisable for a number of shares to be determined by the Company’s board of directors from time to time, upon the achievement by Mayo of specified milestones. The performance warrant provided for termination upon the closing of an initial public offering by the Company. The IPO closed prior to any probable achievement of the specified milestones, therefore, the warrant terminated and the Company did not record any expense for the performance warrant from the date of issuance through the closing of the IPO. 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. The total number of shares of common stock that may be issued under the 2012 Plan was 1,261,836 shares as of December 31, 2013. In May 2014, the Company effected an increase in the number of shares of common stock reserved for issuance under the 2012 Plan to 3,561,836 shares. In December 2014, the Company effected an increase in the number of shares of common stock reserved for issuance under the 2012 Plan to 3,608,029 shares. The total number of shares of common stock that may be issued under the 2012 Plan was 3,608,029 shares as of December 31, 2014. In March 2015, the Company effected an increase in the number of shares of common stock reserved for issuance under the 2012 Plan to 4,309,653 shares. As of December 31, 2015, there were 120,250 shares available for future grant under the 2012 Plan. As required by the 2012 Plan, the exercise price for stock options granted is not to be less than the fair value of common shares as determined by the Company as of the date of grant. The Company values its common stock by taking into consideration its most recently available valuation of common shares performed by management and the board of directors as well as additional factors which may have changed since the date of the most recent contemporaneous valuation through the date of grant. 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, 2015, there were 1,101,500 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. 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, 2015 2014 2013 Risk-free interest rate 1.80 % 1.83 % 1.27 % Expected term (in years) 6.0 6.0 6.0 Expected volatility 81.4 % 83.5 % 85.9 % 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, 2015: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2014 3,579,342 $ 1.38 9.21 $ 59,498 Granted 1,750,124 20.44 Exercised (232,970 ) 0.40 Forfeited (70,250 ) 5.04 Outstanding as of December 31, 2015 5,026,246 $ 8.01 8.70 $ 136,945 Options vested and expected to vest as of December 31, 2015 5,026,246 $ 8.01 8.70 $ 136,945 Options exercisable as of December 31, 2015 1,643,955 $ 1.33 8.13 $ 55,507 The weighted average grant-date fair value of stock options granted during the years ended December 31, 2015, 2014 and 2013 was $14.56, $4.25 and $0.35 per share, respectively. The total intrinsic value of stock options exercised during the year ended December 31, 2015 was $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. During the year ended December 31, 2015, the Company granted performance-based stock options to employees for the purchase of an aggregate of 30,000 shares of common stock with a grant date fair value of $12.89 per share. During the year ended December 31, 2014, the Company granted performance-based stock options to employees for the purchase of an aggregate of 60,000 shares of common stock with a grant date fair value of $3.92 per share. These stock options are exercisable only upon achievement of specified performance targets in each option agreement. As of the years ended December 31, 2015 and 2014, none of these options were exercisable because none of the specified performance targets had been achieved. Because achievement of the specified performance targets was not deemed probable as of the years ended December 31, 2015 and 2014, the Company did not record any expense for these stock options from date of issuance through December 31, 2015. As of December 31, 2015 and 2014, there were outstanding unvested service-based stock options held by non-employees for the purchase of 27,750 and 69,688 shares, respectively, of common stock. Restricted Common Stock The Company has granted restricted common stock with time-based vesting conditions. Unvested shares of restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2015: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted common stock as of December 31, 2014 52,500 $ 0.001 Vested (52,500 ) $ 0.001 Unvested restricted common stock as of December 31, 2015 — $ 0.001 During the year ended December 31, 2013, the Company reacquired, at their original issuance price, 735,000 shares of restricted common stock that were forfeited by former employees. The aggregate intrinsic value of restricted stock awards that vested during the years ended December 31, 2015, 2014 and 2013 was $1,508, $684 and $185, respectively. Stock-based Compensation The Company recorded stock-based compensation expense related to stock options and restricted common stock in the following expense categories of its consolidated statements of operations and comprehensive loss: Year Ended December 31, 2015 2014 2013 Research and development expenses $ 5,297 $ 1,068 $ 177 General and administrative expenses 4,397 1,000 32 $ 9,694 $ 2,068 $ 209 As of December 31, 2015, the Company had an aggregate of $32,665 of unrecognized stock- based compensation cost, which is expected to be recognized over a weighted average period of 3.1 years.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Net Loss per Share	11. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, 2015 2014 2013 Numerator: Net loss $ (54,780 ) $ (16,709 ) $ (6,102 ) Accretion of convertible preferred stock to redemption value - (1,291 ) (875 ) Net loss attributable to common stockholders $ (54,780 ) $ (18,000 ) $ (6,977 ) Denominator: Weighted average common shares outstanding, basic and diluted 23,532,400 6,748,037 6,394,916 Net loss per share attributable to common stockholders, basic and diluted $ (2.33 ) $ (2.67 ) $ (1.09 ) The Company’s potential dilutive securities, which include stock options, unvested restricted common stock, convertible preferred stock and warrants to purchase convertible preferred stock or 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, 2015 2014 2013 Stock options to purchase common stock 5,026,246 3,579,342 1,261,836 Unvested restricted common stock — 52,500 198,750 Warrants for the purchase of convertible preferred stock — 92,127 92,127 Warrants for the purchase of common stock — 738,635 — Convertible preferred stock (as converted to common stock) — 21,478,098 10,478,189 5,026,246 25,940,702 12,030,902

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Leases The Company leases office and laboratory space under an operating lease agreement. The lease expires in January 2018 with no extension periods. The Company does have a right of expansion over the term as additional space becomes available but not an obligation. On February 13, 2015, the Company entered into a sublease for office space with a term expiring in May 2016. On April 1, 2015, the Company entered into a lease for additional office and laboratory space with a term expiring in April 2020. During the years ended December 31, 2015, 2014 and 2013, the Company recognized $1,246, $543 and $152 respectively, of rental expense related to office and laboratory space. 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 is expected to commence in March 2016 and end in November 2023. The Company has the option to extend the lease twice, each for a five-year period. The Company intends to move its corporate headquarters to this location in mid-2016. BMR will contribute a total of $12,509 toward the cost of tenant improvements. BMR’s contributions toward the cost of tenant improvements will be recorded as a lease incentive obligation on our consolidated balance sheet. The lease incentive obligation will be amortized to our consolidated statement of operations as reductions to rent expense over the lease term. As of December 31, 2015, we have recorded a lease incentive obligation of $586. Future minimum lease payments for these operating leases as of December 31, 2015 are as follows: Year Ending December 31, 2016 $ 1,274 2017 1,162 2018 643 2019 478 2020 80 2021 and thereafter - Total $ 3,637 Lease Commitments Minimum lease payments due under the Sidney Street lease are $911 during the year ending December 31, 2016, $5,528 during the year ending December 31, 2017, $5,694 during the year ending December 31, 2018, $5,864 during the year ending December 31, 2019, $6,040 during the year ending December 31, 2020 and $17,752 thereafter. These amounts are not included within the future minimum lease payments table above as our lease with BMR had not commenced as of December 31, 2015. 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, 2015 or 2014.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	13. Income Taxes During the years ended December 31, 2015, 2014 and 2013, 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, 2015 2014 2013 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% Research and development tax credits (5.7 ) (3.8 ) (6.1 ) State taxes, net of federal benefit (5.3 ) (5.3 ) (5.3 ) Stock-based compensation 4.1 2.3 1.2 Revaluation of preferred stock warrant liability — 3.3 — Other 0.3 0.2 — Change in deferred tax asset valuation allowance 40.6 37.3 44.2 Effective income tax rate — % — % — % Net deferred tax assets as of December 31, 2015 and 2014 consisted of the following: December 31, 2015 2014 Deferred tax assets: Net operating loss carryforwards $ 25,293 $ 7,946 Research and development tax credit carryforwards 4,180 1,055 Capitalized organization costs 527 571 Stock-based compensation expense 1,897 436 Charitable Contributions 5 — Accrued expenses 977 447 Capitalized research and development expenses 126 136 Total deferred tax assets $ 33,005 10,591 Deferred tax liabilities: Depreciation and amortization (228 ) (69 ) Total deferred tax liabilities (228 ) (69 ) Valuation allowance $ (32,777 ) (10,522 ) Net deferred tax assets $ — $ — As of December 31, 2015, the Company had net operating loss carryforwards for federal and state income tax purposes of $65,401 and $64,266, respectively, which both begin to expire in 2031. As of December 31, 2015, the Company also had available research and development tax credit carryforwards for federal and state income tax purposes of $3,045 and $1,331, respectively, which begin to expire in 2031 and 2027, respectively. 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. At December 31, 2015, $856 of the federal net operating loss carryforwards, and $856 of the state net operating loss carryforwards relate to excess stock based compensation tax benefits for which the benefit will be recorded to additional paid-in capital when recognized. 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, 2015 and 2014. 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, 2015, 2014 and 2013 related primarily to the increases in net operating loss carryforwards, research and development tax credit carryforwards and stock-based compensation were as follows: Year Ended December 31, 2015 2014 2013 Valuation allowance at beginning of year $ (10,522 ) $ (4,294 ) $ (1,599 ) Decreases recorded as benefit to income tax provision — — — Increases recorded to income tax provision (22,255 ) (6,228 ) (2,695 ) Valuation allowance as of end of year $ (32,777 ) $ (10,522 ) $ (4,294 )

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Selected Quarterly Financial Data (Unaudited)	14. Selected Quarterly Financial Data (Unaudited) The following table contains quarterly financial information for 2015 and 2014. 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. 2015 First Quarter Second Quarter Third Quarter Fourth Quarter Total Total operating expenses $ 8,167 $ 12,340 $ 14,561 $ 19,788 $ 54,856 Loss from operations (8,167 ) (12,340 ) (14,561 ) (19,788 ) (54,856 ) Net loss (7,971 ) (12,555 ) (14,620 ) (19,634 ) (54,780 ) Net loss per share applicable to common stockholders - basic and diluted $ (1.15 ) $ (1.45 ) $ (0.38 ) $ (0.50 ) $ (2.33 ) 2014 First Quarter Second Quarter Third Quarter Fourth Quarter Total Total operating expenses $ 1,672 $ 2,618 $ 3,579 $ 7,213 $ 15,082 Loss from operations (1,672 ) (2,618 ) (3,579 ) (7,213 ) (15,082 ) Net loss (1,689 ) (2,677 ) (4,161 ) (8,182 ) (16,709 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.29 ) $ (0.45 ) $ (0.68 ) $ (1.24 ) $ (2.67 )

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2015
Related Party Transactions [Abstract]
Related Party Transactions	15. 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, 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 $502, $454 and $391 during the years ended December 31, 2015, 2014 and 2013, respectively. There were no amounts due to Flagship Ventures Management, Inc. related to the services agreement as of December 31, 2015 and 2014.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
401(k) Savings Plan	16. 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. Company contributions to the plan may be made at the discretion of the Company’s board of directors. The Company has not made any contributions to date under the 401(k) Savings Plan as of December 31, 2015.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	17. Subsequent Events In January 2016 the Company entered into a Collaboration and License Agreement (the “License Agreement”) with Nestec Ltd. (“NHS”) for the development and commercialization of certain of the Company’s product candidates in development for the treatment and management of CDI and IBD, including ulcerative colitis and Crohn’s disease. NHS is a related party to the Company since an affiliate of NHS is one of the Company’s stockholders. 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”) and is expected to provide financial support for the Company’s ongoing research and development. The Company has retained full commercial rights to the Company’s entire portfolio of product candidates with respect to the United States and Canada. Under the License Agreement, the Company granted to NHS (i) an exclusive, royalty-bearing license to develop and commercialize, in the Licensed Territory, certain products based on the Company’s 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”) and (ii) a non-exclusive license to develop, make and export NHS Collaboration Products worldwide solely for the commercialization in the Licensed Territory. In exchange for the license, NHS is obligated to pay the Company an upfront cash payment of $120,000, which the Company received in February 2016. NHS has 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. Additionally, NHS has agreed to pay the Company up to $660,000 for the achievement of certain development and regulatory milestones and up to an aggregate of $1,125,000 for the achievement of certain commercial milestones related to the sales of NHS Collaboration Products.

Summary of Significant Accoun26

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
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, the accrual of research and development expenses and the valuation of common stock, stock-based awards and the preferred stock warrant liability. 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 commercial paper 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 $1,539 as of December 31, 2015 and $139 as of December 31, 2014 in a separate restricted bank account as a security deposit for the lease of the Company’s facilities and as collateral for the Company’s credit card program with Comerica Bank. 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 one accredited financial institution, 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, investments and preferred stock warrant liability are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s cash, prepaid expenses, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities. The carrying value of the Company’s outstanding debt as of December 31, 2014 approximates fair value based on the variable interest rate for the borrowings outstanding as well as short duration of the term of the note. The fair value of the outstanding debt was estimated using a discounted cash flow analysis based on current market interest rates for debt issuances with similar remaining years to maturity, adjusted for credit risk, which represents a Level 3 measurement.
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. As of December 31, 2015 and 2014 the Company had recorded $0 and $1,684, respectively, of deferred offering costs in contemplation of the IPO. On July 1, 2015, the Company reclassified $3,748 of deferred offering costs to additional paid in capital as a reduction of the proceeds from the IPO.
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, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options and restricted stock awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. 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 compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to stock- based compensation expense in future periods. 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.
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. No revenue has been generated since inception, and 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. There was no difference between net loss and comprehensive loss for the years ended December 31, 2014 and 2013. For the year ended December 31, 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 the 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 No. 2014-09, Revenue from Contracts with Customers Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, In April 2015, the FASB issued ASU 2015-03, Simplifying the Presentation of Debt Issuance Costs In May 2015, the FASB issued ASU No. 2015-07, Fair Value Measurement (Topic 820): Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent). The new standard removes the requirement to categorize within the fair value hierarchy all investments for which fair value is measured using the net asset value per share practical expedient. The new standard will be effective for us on January 1, 2016. Early application is permitted. The Company believes the adoption of this standard will not have a material impact on its financial statements. In November 2015, the FASB issued ASU 2015-17, “Income Taxes (Topic 740): Balance Sheet Classification of Deferred Taxes,” to simplify the presentation of deferred income taxes by requiring that deferred tax liabilities and assets be classified as noncurrent on the balance sheet. This update is effective for annual reporting periods beginning after December 31, 2016, including interim periods within those annual periods, and early adoption is permitted. Accordingly, the Company elected to early adopt ASU 2015-17 for the year ended December 31, 2015. There was no impact to the consolidated financial statements as a result of this change.

Fair Value of Financial Asset27

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 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, 2015 Using: Level 1 Level 2 Level 3 Total Assets: Cash Equivalents $ — $ 23,125 $ — $ 23,125 Repurchase Agreements — 20,000 — 20,000 Investments: Commercial Paper — 64,820 — 64,820 Corporate Bonds — 46,490 — 46,490 Government Securities — 15,819 — 15,819 Treasury Bonds — 4,020 — 4,020 $ $ 174,274 $ $ 174,274 Fair Value Measurements as of December 31, 2014 Using: Level 1 Level 2 Level 3 Total Liabilities: Liability for preferred stock warrant $ — $ — $ 1,582 $ 1,582 $ — $ — $ 1,582 $ 1,582

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
Schedule of Fair Value of Available-for-Sale Investments by Type of Security	As of December 31, 2015, the fair value of available-for-sale investments by type of security was as follows: December 31, 2015 Amortized Cost Gross Unrealized Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 64,733 $ 87 $ — $ 64,820 Corporate Bonds 46,538 — (48 ) 46,490 Government Securities 15,823 — (4 ) 15,819 Treasury Bonds 4,022 — (2 ) 4,020 $ 131,116 $ 87 $ (54 ) $ 131,149

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following: December 31, 2015 December 31, 2014 Laboratory equipment $ 4,370 $ 1,260 Computer equipment 408 115 Furniture and office equipment 285 58 Leasehold improvements 1,856 114 Construction in progress 1,843 — 8,762 1,547 Less: Accumulated depreciation and amortization (1,011 ) (283 ) $ 7,751 $ 1,264

Accrued Expenses and Other Cu30

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2015
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: December 31, 2015 December 31, 2014 Development and clinical manufacturing costs $ 1,436 $ 598 Payroll and payroll-related costs 2,756 547 Professional fees 184 314 Facility and other 1,147 278 $ 5,523 $ 1,737

Preferred Stock Warrant Liabi31

Preferred Stock Warrant Liability (Tables)	12 Months Ended
	Dec. 31, 2015
Class Of Stock Disclosures [Abstract]
Summary of Assumptions and Inputs Used in Determining Fair Value of Preferred Stock Warrant Liability Valued Using Black-Scholes Option-Pricing Model	The following assumptions and inputs were used in determining the fair value of the preferred stock warrant liability valued using the Black-Scholes option-pricing model: Year Ended December 31, 2015 2014 2013 Risk-free interest rate 2.40 % 2.17 % 3.20 % Expected term (in years) 8.2 8.7 9.7 Expected volatility 91.2 % 84.0 % 86.0 % Expected dividend yield 0 % 0 % 0 % Fair value of Series A-2 convertible preferred stock $ 17.26 $ 17.18 $ 2.07
Summary of Rollforward of Fair Value of Preferred Stock Warrant Liability	The following table provides a rollforward of the fair value of the Company’s preferred stock warrant liability: Fair Value Balance as of December 31, 2012 $ — Issuance of Series A-2 preferred stock warrant 156 Loss on revaluation 8 Balance as of December 31, 2013 164 Loss on revaluation 1,418 Balance as of December 31, 2014 $ 1,582 Loss on revaluation 7 Reclassification to stockholders’ equity (1,589 ) Balance as of December 31, 2015 $ —

Stockholders_ Equity (Deficit32

Stockholders’ Equity (Deficit) Common Stock (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 2014 2013 Risk-free interest rate 1.80 % 1.83 % 1.27 % Expected term (in years) 6.0 6.0 6.0 Expected volatility 81.4 % 83.5 % 85.9 % 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, 2015: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2014 3,579,342 $ 1.38 9.21 $ 59,498 Granted 1,750,124 20.44 Exercised (232,970 ) 0.40 Forfeited (70,250 ) 5.04 Outstanding as of December 31, 2015 5,026,246 $ 8.01 8.70 $ 136,945 Options vested and expected to vest as of December 31, 2015 5,026,246 $ 8.01 8.70 $ 136,945 Options exercisable as of December 31, 2015 1,643,955 $ 1.33 8.13 $ 55,507
Summary of Restricted Stock Activity	The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2015: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted common stock as of December 31, 2014 52,500 $ 0.001 Vested (52,500 ) $ 0.001 Unvested restricted common stock as of December 31, 2015 — $ 0.001
Summary of Stock Based Compensation Expense	The Company recorded stock-based compensation expense related to stock options and restricted common stock in the following expense categories of its consolidated statements of operations and comprehensive loss: Year Ended December 31, 2015 2014 2013 Research and development expenses $ 5,297 $ 1,068 $ 177 General and administrative expenses 4,397 1,000 32 $ 9,694 $ 2,068 $ 209

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 2014 2013 Numerator: Net loss $ (54,780 ) $ (16,709 ) $ (6,102 ) Accretion of convertible preferred stock to redemption value - (1,291 ) (875 ) Net loss attributable to common stockholders $ (54,780 ) $ (18,000 ) $ (6,977 ) Denominator: Weighted average common shares outstanding, basic and diluted 23,532,400 6,748,037 6,394,916 Net loss per share attributable to common stockholders, basic and diluted $ (2.33 ) $ (2.67 ) $ (1.09 )
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, 2015 2014 2013 Stock options to purchase common stock 5,026,246 3,579,342 1,261,836 Unvested restricted common stock — 52,500 198,750 Warrants for the purchase of convertible preferred stock — 92,127 92,127 Warrants for the purchase of common stock — 738,635 — Convertible preferred stock (as converted to common stock) — 21,478,098 10,478,189 5,026,246 25,940,702 12,030,902

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 are as follows: Year Ending December 31, 2016 $ 1,274 2017 1,162 2018 643 2019 478 2020 80 2021 and thereafter - Total $ 3,637

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 2014 2013 Federal statutory income tax rate (34.0 )% (34.0 )% (34.0 )% Research and development tax credits (5.7 ) (3.8 ) (6.1 ) State taxes, net of federal benefit (5.3 ) (5.3 ) (5.3 ) Stock-based compensation 4.1 2.3 1.2 Revaluation of preferred stock warrant liability — 3.3 — Other 0.3 0.2 — Change in deferred tax asset valuation allowance 40.6 37.3 44.2 Effective income tax rate — % — % — %
Schedule of Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2015 and 2014 consisted of the following: December 31, 2015 2014 Deferred tax assets: Net operating loss carryforwards $ 25,293 $ 7,946 Research and development tax credit carryforwards 4,180 1,055 Capitalized organization costs 527 571 Stock-based compensation expense 1,897 436 Charitable Contributions 5 — Accrued expenses 977 447 Capitalized research and development expenses 126 136 Total deferred tax assets $ 33,005 10,591 Deferred tax liabilities: Depreciation and amortization (228 ) (69 ) Total deferred tax liabilities (228 ) (69 ) Valuation allowance $ (32,777 ) (10,522 ) 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, 2015, 2014 and 2013 related primarily to the increases in net operating loss carryforwards, research and development tax credit carryforwards and stock-based compensation were as follows: Year Ended December 31, 2015 2014 2013 Valuation allowance at beginning of year $ (10,522 ) $ (4,294 ) $ (1,599 ) Decreases recorded as benefit to income tax provision — — — Increases recorded to income tax provision (22,255 ) (6,228 ) (2,695 ) Valuation allowance as of end of year $ (32,777 ) $ (10,522 ) $ (4,294 )

Selected Quarterly Financial 36

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Schedule of Quarterly Financial Information	The following table contains quarterly financial information for 2015 and 2014. 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. 2015 First Quarter Second Quarter Third Quarter Fourth Quarter Total Total operating expenses $ 8,167 $ 12,340 $ 14,561 $ 19,788 $ 54,856 Loss from operations (8,167 ) (12,340 ) (14,561 ) (19,788 ) (54,856 ) Net loss (7,971 ) (12,555 ) (14,620 ) (19,634 ) (54,780 ) Net loss per share applicable to common stockholders - basic and diluted $ (1.15 ) $ (1.45 ) $ (0.38 ) $ (0.50 ) $ (2.33 ) 2014 First Quarter Second Quarter Third Quarter Fourth Quarter Total Total operating expenses $ 1,672 $ 2,618 $ 3,579 $ 7,213 $ 15,082 Loss from operations (1,672 ) (2,618 ) (3,579 ) (7,213 ) (15,082 ) Net loss (1,689 ) (2,677 ) (4,161 ) (8,182 ) (16,709 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.29 ) $ (0.45 ) $ (0.68 ) $ (1.24 ) $ (2.67 )

Nature of the Business and Ba37

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jul. 02, 2015	Jun. 26, 2015	Dec. 31, 2015	Dec. 31, 2014	Jul. 01, 2015
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporated year			Oct. 31, 2010
Entity incorporated state name			State of Delaware
Accumulated deficit			$ 82,612	$ 27,832
Cash, cash equivalents and investments			$ 205,082
Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock issued and sold	8,545,138		8,545,138	6,563
Net proceeds from issuance of common stock from IPO after deducting underwriting discounts and commissions	$ 139,267
Conversion of redeemable convertible preferred stock into common stock		22,866,987	22,866,987
Initial Public Offering ("IPO") [Member] | Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Public offering price per share		$ 18			$ 18
Conversion of redeemable convertible preferred stock into common stock		22,866,987
Underwriters Option to Purchase Additional Shares [Member] | Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock issued and sold	1,114,583

Summary of Significant Accoun38

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	Jul. 02, 2015	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Line Items]
Maximum maturity days for cash equivalents		90 days
Restricted cash		$ 1,539,000	$ 139,000
Deferred offering costs			1,684,000
Reclassification of deferred offering costs to additional paid in capital	$ 3,748,000
Impairment losses on long lived assets		$ 0
Minimum percentage of likelihood of tax benefit being realized upon settlement		50.00%
Revenue		$ 0
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
Initial Public Offering ("IPO") [Member]
Accounting Policies [Line Items]
Deferred offering costs		$ 0	$ 1,684,000

Fair Value of Financial Asset39

Fair Value of Financial Assets and Liability - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Measurements, Recurring [Member]
Assets
Cash Equivalents	$ 23,125
Repurchase Agreements	20,000
Investments:
Total assets	174,274
Liabilities:
Liability for preferred stock warrant		$ 1,582
Total liabilities		1,582
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets
Cash Equivalents	23,125
Repurchase Agreements	20,000
Investments:
Total assets	174,274
Fair Value, Inputs, Level 3 [Member]
Liabilities:
Liability for preferred stock warrant		1,582
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities:
Liability for preferred stock warrant		1,582
Total liabilities		$ 1,582
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	64,820
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	64,820
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	46,490
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	46,490
Government Securities [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	15,819
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	15,819
Treasury Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	4,020
Treasury Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	$ 4,020

Fair Value of Financial Asset40

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Jul. 01, 2015	Jun. 26, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Maximum maturity days for cash equivalents	90 days
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
Common Stock [Member] | Initial Public Offering ("IPO") [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Public offering price per share			$ 18	$ 18
Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability for preferred stock warrant		1,582,000
Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability for preferred stock warrant		1,582,000
Other assets fair value	0	0
Other liability fair value	$ 0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability for preferred stock warrant		$ 1,582,000

Investments - Schedule of Fair

Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail) $ in Thousands	Dec. 31, 2015USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 131,116
Gross Unrealized Gain	87
Gross Unrealized Loss	(54)
Fair Value	131,149
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	64,733
Gross Unrealized Gain	87
Fair Value	64,820
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	46,538
Gross Unrealized Loss	(48)
Fair Value	46,490
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	15,823
Gross Unrealized Loss	(4)
Fair Value	15,819
Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	4,022
Gross Unrealized Loss	(2)
Fair Value	$ 4,020

Investments - Additional Inform

Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015
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, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 8,762	$ 1,547
Less: Accumulated depreciation and amortization	(1,011)	(283)
Property and equipment, net	7,751	1,264
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,370	1,260
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	408	115
Furniture and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	285	58
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,856	$ 114
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,843

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 728	$ 190	$ 88

Accrued Expenses and Other Cu45

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Payables And Accruals [Abstract]
Development and clinical manufacturing costs	$ 1,436	$ 598
Payroll and payroll-related costs	2,756	547
Professional fees	184	314
Facility and other	1,147	278
Total Accrued Expenses and Other Current Liabilities	$ 5,523	$ 1,737

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail) - Comerica [Member] - USD ($)	Sep. 17, 2015	Sep. 09, 2013	Aug. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2014
Debt Instrument [Line Items]
Additional interest expense				$ 96,000	$ 66,000
Unamortized debt discount				0	96,000	$ 96,000
Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Borrowings amount		$ 3,000,000
Amount received from borrowings		$ 1,000,000	$ 2,000,000			$ 3,000,000
Principal repayments amount				900,000	$ 400,000
Payments of Debt	$ 1,765,000
Extinguishment amount of outstanding principal	1,700,000
Final fees and accrued interest	$ 65,000
Loss on extinguishment debt				$ 140,000

Preferred Stock Warrant Liabi47

Preferred Stock Warrant Liability - Additional Information (Detail) - USD ($)	Aug. 17, 2015	Sep. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jul. 01, 2015	Jun. 26, 2015	Dec. 31, 2012
Class of Stock [Line Items]
Fair value of warrant issuance			$ 0	$ 1,582,000	$ 164,000			$ 0
Loss from revaluation of preferred stock warrant liability			7,000	1,418,000	8,000
Common Stock [Member]
Class of Stock [Line Items]
Warrants exercise price per share	$ 1.78
Proceeds from exercise of warrant	$ 164,000
Common Stock [Member] | Initial Public Offering ("IPO") [Member]
Class of Stock [Line Items]
Public offering price per share						$ 18	$ 18
Series A-2 Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Warrants issued to purchase of stock		92,127
Warrants exercise price per share		$ 1.78
Class of warrant or right, contractual term		10 years
Fair value of warrant issuance		$ 156,000	$ 17,260	$ 17,180	$ 2,070

Preferred Stock Warrant Liabi48

Preferred Stock Warrant Liability - Summary of Assumptions and Inputs Used in Determining Fair Value of Preferred Stock Warrant Liability Valued Using Black-Scholes Option-Pricing Model (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	2.40%	2.17%	3.20%
Expected term (in years)	8 years 2 months 12 days	8 years 8 months 12 days	9 years 8 months 12 days
Expected volatility	91.20%	84.00%	86.00%
Expected dividend yield	0.00%	0.00%	0.00%
Fair value of Series A-2 convertible preferred stock	$ 0	$ 1,582,000	$ 164,000		$ 0
Series A-2 Convertible Preferred Stock [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value of Series A-2 convertible preferred stock	$ 17,260	$ 17,180	$ 2,070	$ 156,000

Preferred Stock Warrant Liabi49

Preferred Stock Warrant Liability - Summary of Rollforward of Fair Value of Preferred Stock Warrant Liability (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Beginning balance	$ 1,582,000	$ 164,000	$ 0
Loss from revaluation of preferred stock warrant liability	7,000	1,418,000	8,000
Reclassification to stockholders’ equity	(1,589,000)
Ending balance	0	1,582,000	164,000
Series A-2 Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Beginning balance	17,180	2,070
Issuance of Series A-2 preferred stock warrant			156,000
Ending balance	$ 17,260	$ 17,180	$ 2,070

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - $ / shares	Jun. 26, 2015	Dec. 31, 2015	Jul. 01, 2015	Dec. 31, 2014
Common Stock [Member]
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock into common stock	22,866,987	22,866,987
Convertible Preferred Stock Series A, A-2, B, C, D and D-1 [Member]
Class of Stock [Line Items]
Convertible preferred stock, shares authorized		10,000,000	10,000,000	24,348,003
Convertible preferred stock, par value		$ 0.001	$ 0.001	$ 0.001
Conversion of redeemable convertible preferred stock into common stock	22,866,987	(22,866,987)

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jul. 02, 2015	Jun. 26, 2015	Dec. 31, 2015	Dec. 31, 2014	Jul. 01, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized			200,000,000	38,000,000
Common stock, par value			$ 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	6,563
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			$ 18
Conversion of redeemable convertible preferred stock into common stock		22,866,987
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 (Deficit52

Stockholders' Equity (Deficit) Common Stock - Common Stock Warrants - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Aug. 17, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Research and development expense for fair value of the warrant		$ 38,095,000	$ 10,718,000	$ 4,805,000
Expected volatility		91.20%	84.00%	86.00%
Risk-free interest rate		2.40%	2.17%	3.20%
Expected term (in years)		8 years 2 months 12 days	8 years 8 months 12 days	9 years 8 months 12 days
Expected dividend yield		0.00%	0.00%	0.00%
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants exercise price per share					$ 1.78
License Agreement Terms [Member] | Mayo Foundation [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants exercise price per share	$ 0.01
Research and development expense for fair value of the warrant	$ 317,000
Expected volatility	86.00%
Risk-free interest rate	2.30%
Expected term (in years)	7 years
Expected dividend yield	0.00%
Warrant exercised date	Apr. 29, 2015
License Agreement Terms [Member] | Mayo Foundation [Member] | Performance Warrant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants issued to purchase shares of common stock		284,090
Warrant expense	$ 0
License Agreement Terms [Member] | Mayo Foundation [Member] | Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants issued to purchase shares of common stock	454,545

Stockholders' Equity (Deficit53

Stockholders' Equity (Deficit) Common Stock - 2012 Stock Incentive plan - Additional Information (Detail) - 2012 Stock Incentive Plan [Member] - shares	12 Months Ended
	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	May. 31, 2014	Dec. 31, 2013
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	120,250
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock that may be issued		4,309,653	3,608,029	3,561,836	1,261,836
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 (Deficit54

Stockholders' Equity (Deficit) Common Stock - 2015 Incentive Award Plan - Additional Information (Detail) - 2015 Incentive Award Plan [Member] - shares	Jun. 16, 2015	Dec. 31, 2015
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		1,101,500

Stockholders' Equity (Deficit55

Stockholders' Equity (Deficit) Common Stock - 2015 Employee Stock Purchase Plan - Additional Information (Detail) - 2015 Employee Stock Purchase Plan [Member]	Jun. 16, 2015shares
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
Percentage of common stock shares outstanding	1.00%

Stockholders' Equity (Deficit56

Stockholders' Equity (Deficit) Common Stock - Summary of Estimated Fair Value of Employee Stock Options (Detail) - Stock Option Valuation [Member]	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.80%	1.83%	1.27%
Expected term (in years)	6 years	6 years	6 years
Expected volatility	81.40%	83.50%	85.90%
Expected dividend yield	0.00%	0.00%	0.00%

Stockholders' Equity (Deficit57

Stockholders' Equity (Deficit) Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Number of shares, Beginning Balance	3,579,342
Number of shares, Granted	1,750,124
Number of shares, Exercised	(232,970)
Number of shares, Forfeited	(70,250)
Number of shares, Ending Balance	5,026,246	3,579,342
Number of shares, Options vested and expected to vest	5,026,246
Number of shares, Options exercisable	1,643,955
Weighted Average Exercise Price, Beginning Balance	$ 1.38
Weighted Average Exercise Price, Granted	20.44
Weighted Average Exercise Price, Exercised	0.40
Weighted Average Exercise Price, Forfeited	5.04
Weighted Average Exercise Price, Ending Balance	8.01	$ 1.38
Weighted Average Exercise Price, Options vested and expected to vest	8.01
Weighted Average Exercise Price, Options exercisable	$ 1.33
Weighted Average Remaining Contractual Term, Outstanding	8 years 8 months 12 days	9 years 2 months 16 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest	8 years 8 months 12 days
Weighted Average Remaining Contractual Term, Options exercisable	8 years 1 month 17 days
Aggregate Intrinsic Value	$ 136,945	$ 59,498
Aggregate Intrinsic Value, Options vested and expected to vest	136,945
Aggregate Intrinsic Value, Options exercisable	$ 55,507

Stockholders' Equity (Deficit58

Stockholders' Equity (Deficit) Common Stock - Stock Options - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options	$ 14.56	$ 4.25	$ 0.35
Intrinsic Value, Stock options exercised	$ 4,125,000
Expense for stock options	$ 9,694,000	$ 2,068,000	$ 209,000
Outstanding unvested service-based stock options	27,750	69,688
Stock Option [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-Based stock options granted	30,000	60,000
Grant date fair value of options	$ 12.89	$ 3.92
Stock option exercisable	0	0
Expense for stock options	$ 0	$ 0

Stockholders' Equity (Deficit59

Stockholders' Equity (Deficit) Common Stock - Summary of Restricted Stock Activity (Detail) - Restricted Common Stock [Member]	12 Months Ended
	Dec. 31, 2015$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested restricted common stock, Beginning balance | shares	52,500
Number of Shares, Vested | shares	(52,500)
Weighted Average Grant Date Fair Value, Unvested restricted common stock, Beginning balance	$ 0.001
Weighted Average Grant Date Fair Value, Vested	0.001
Weighted Average Grant Date Fair Value, Unvested restricted common stock, Ending balance	$ 0.001

Stockholders' Equity (Deficit60

Stockholders' Equity (Deficit) Common Stock - Restricted Common Stock - Additional Information (Detail) - Restricted Common Stock [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of restricted common stock forfeited			735,000
Aggregate intrinsic value, vested	$ 1,508	$ 684	$ 185

Stockholders' Equity (Deficit61

Stockholders' Equity (Deficit) Common Stock - Summary of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 9,694	$ 2,068	$ 209
Research and development expenses [Member] | Stock Options and Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	5,297	1,068	177
General and administrative expenses [Member] | Stock Options and Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 4,397	$ 1,000	$ 32

Stockholders' Equity (Deficit62

Stockholders' Equity (Deficit) Common Stock - Stock Based Compensation - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)
Equity [Abstract]
Unrecognized stock- based compensation cost	$ 32,665
Weighted average period over which unrecognized compensation cost is expected to be recognized	3 years 1 month 6 days

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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net loss	$ (19,634)	$ (14,620)	$ (12,555)	$ (7,971)	$ (8,182)	$ (4,161)	$ (2,677)	$ (1,689)	$ (54,780)	$ (16,709)	$ (6,102)
Accretion of convertible preferred stock to redemption value										(1,291)	(875)
Net loss attributable to common stockholders									$ (54,780)	$ (18,000)	$ (6,977)
Denominator:
Weighted average common shares outstanding, basic and diluted									23,532,400	6,748,037	6,394,916
Net loss per share attributable to common stockholders, basic and diluted	$ (0.50)	$ (0.38)	$ (1.45)	$ (1.15)	$ (1.24)	$ (0.68)	$ (0.45)	$ (0.29)	$ (2.33)	$ (2.67)	$ (1.09)

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, 2015	Dec. 31, 2014	Dec. 31, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	5,026,246	25,940,702	12,030,902
Convertible Preferred Stock Series A, A-2, B, C, D and D-1 [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share		21,478,098	10,478,189
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	5,026,246	3,579,342	1,261,836
Unvested Restricted 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		52,500	198,750
Warrants for the Purchase of Convertible Preferred 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		92,127	92,127
Warrants for the Purchase of 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		738,635

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Nov. 11, 2015USD ($)ft²	Apr. 02, 2015	Feb. 13, 2015	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Other Commitments [Line Items]
Operating lease agreement expiration date				Jan. 31, 2018
Lease incentive obligation				$ 586,000
Future minimum lease payments in 2016				1,274,000
Future minimum lease payments in 2017				1,162,000
Future minimum lease payments in 2018				643,000
Future minimum lease payments in 2019				478,000
Future minimum lease payments in 2020				80,000
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 is expected to commence in March 2016 and end in November 2023. The Company has the option to extend the lease twice, each for a five-year period. The Company intends to move its corporate headquarters to this location in mid-
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				$ 586,000
Future minimum lease payments in 2016				911,000
Future minimum lease payments in 2017				5,528,000
Future minimum lease payments in 2018				5,694,000
Future minimum lease payments in 2019				5,864,000
Future minimum lease payments in 2020				6,040,000
Future minimum lease payments Thereafter				17,752,000
Office Space [Member]
Other Commitments [Line Items]
Operating lease agreement expiration date			May 31, 2016
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				$ 1,246,000	$ 543,000	$ 152,000

Commitments and Contingencies66

Commitments and Contingencies - Summary Of Future Minimum Lease Payments For Operating Leases (Detail) $ in Thousands	Dec. 31, 2015USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,016	$ 1,274
2,017	1,162
2,018	643
2,019	478
2,020	80
Total	$ 3,637

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Line Items]
Income tax benefit	$ 0	$ 0	$ 0
Federal [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 65,401,000
Net operating loss carry forwards, expiration year, start	2,031
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		$ 3,045,000
State [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 64,266,000
Net operating loss carry forwards, expiration year, start	2,031
Research and development tax credit carryforwards, expiration year, start		2,027
State [Member] | Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Research and development tax credit carryforwards		$ 1,331,000
Stock Based Compensation Tax Benefit | Federal [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 856,000
Stock Based Compensation Tax Benefit | State [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 856,000

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, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(34.00%)	(34.00%)	(34.00%)
Research and development tax credits	(5.70%)	(3.80%)	(6.10%)
State taxes, net of federal benefit	(5.30%)	(5.30%)	(5.30%)
Stock-based compensation	4.10%	2.30%	1.20%
Revaluation of preferred stock warrant liability		3.30%
Other	0.30%	0.20%
Change in deferred tax asset valuation allowance	40.60%	37.30%	44.20%
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, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred tax assets:
Net operating loss carryforwards	$ 25,293	$ 7,946
Research and development tax credit carryforwards	4,180	1,055
Capitalized organization costs	527	571
Stock-based compensation expense	1,897	436
Charitable Contributions	5
Accrued expenses	977	447
Capitalized research and development expenses	126	136
Total deferred tax assets	33,005	10,591
Deferred tax liabilities:
Depreciation and amortization	(228)	(69)
Total deferred tax liabilities	(228)	(69)
Valuation allowance	(32,777)	(10,522)	$ (4,294)	$ (1,599)
Net deferred tax assets	$ 0	$ 0

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, 2015	Dec. 31, 2014	Dec. 31, 2013
Valuation Allowance [Abstract]
Valuation allowance at beginning of year	$ (10,522)	$ (4,294)	$ (1,599)
Increases recorded to income tax provision	(22,255)	(6,228)	(2,695)
Valuation allowance as of end of year	$ (32,777)	$ (10,522)	$ (4,294)

Selected Quarterly Financial 71

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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Abstract]
Total operating expenses	$ 19,788	$ 14,561	$ 12,340	$ 8,167	$ 7,213	$ 3,579	$ 2,618	$ 1,672	$ 54,856	$ 15,082	$ 6,052
Loss from operations	(19,788)	(14,561)	(12,340)	(8,167)	(7,213)	(3,579)	(2,618)	(1,672)	(54,856)	(15,082)	(6,052)
Net loss	$ (19,634)	$ (14,620)	$ (12,555)	$ (7,971)	$ (8,182)	$ (4,161)	$ (2,677)	$ (1,689)	$ (54,780)	$ (16,709)	$ (6,102)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.50)	$ (0.38)	$ (1.45)	$ (1.15)	$ (1.24)	$ (0.68)	$ (0.45)	$ (0.29)	$ (2.33)	$ (2.67)	$ (1.09)

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - Flagship Ventures Management, Inc. [Member] - Services Agreement [Member] - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Related Party Transaction [Line Items]
Payments under agreements with related party	$ 502,000	$ 454,000	$ 391,000
Due to related party	$ 0	$ 0

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Events [Member] - Nestlé Health Science [Member] - USD ($) $ in Thousands	1 Months Ended
	Feb. 29, 2016	Jan. 31, 2016
Subsequent Event [Line Items]
Upfront cash payment	$ 120,000
Development, approval and commercial milestones		$ 660,000
Sales based milestones		$ 1,125,000