CytomX Therapeutics (CTMX) 10-K 7 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2015	Mar. 02, 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	CTMX
Entity Registrant Name	CytomX Therapeutics, Inc.
Entity Central Index Key	1,501,989
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$ 0
Entity Common Stock, Shares Outstanding		36,077,873

BALANCE SHEETS

BALANCE SHEETS - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 59,822,000	$ 64,396,000
Restricted cash		100,000
Short-term investments	126,889,000	0
Accounts receivable	744,000	1,875,000
Prepaid expenses and other current assets	2,299,000	482,000
Total current assets	189,754,000	66,853,000
Property and equipment, net	3,481,000	3,018,000
Intangible assets	1,750,000	1,750,000
Goodwill	949,000	949,000
Restricted cash	917,000
Other assets	364,000	492,000
Total assets	197,215,000	73,062,000
Current liabilities:
Accounts payable	4,697,000	1,919,000
Accrued liabilities	4,912,000	1,695,000
Deferred revenues, current portion	6,130,000	6,130,000
Long-term debt, current portion		1,419,000
Total current liabilities	15,739,000	11,163,000
Long-term debt, net of current portion		1,568,000
Deferred revenue, net of current portion	54,703,000	60,833,000
Convertible preferred stock warrant liability		186,000
Convertible preferred stock liability		395,000
Deferred tax liability	507,000	499,000
Other long-term liabilities	198,000	249,000
Total liabilities	$ 71,147,000	$ 74,893,000
Commitments and contingencies (Note 11)
Redeemable convertible preferred stock, $0.00001 par value – 0 and 21,759,654 shares authorized at December 31, 2015 and 2014, respectively; 0 and 18,458,289 shares issued and outstanding at December 31, 2015 and 2014		$ 76,710,000
Stockholders' equity (deficit)
Preferred stock, $0.00001 par value; 10,000,000 shares authorized at December 31, 2015; no shares issued and outstanding at December 31, 2015 and 2014, respectively
Common stock, $0.00001 par value; 75,000,000 and 28,572,789 shares authorized at December 31, 2015 and 2014, respectively; 36,033,209 and 996,520 shares issued and outstanding at December 31, 2015 and 2014, respectively	$ 1,000	$ 1,000
Stockholders notes receivable	(78,000)	(404,000)
Additional paid-in capital	243,687,000
Accumulated other comprehensive loss	(76,000)
Accumulated deficit	(117,466,000)	(78,138,000)
Total stockholders' equity (deficit)	126,068,000	(78,541,000)
Total liabilities, redeemable convertible preferred stock, convertible preferred stock and stockholders' equity (deficit)	$ 197,215,000	73,062,000
Redeemable Convertible Preferred Stock
Current liabilities:
Redeemable convertible preferred stock, $0.00001 par value – 0 and 21,759,654 shares authorized at December 31, 2015 and 2014, respectively; 0 and 18,458,289 shares issued and outstanding at December 31, 2015 and 2014		76,236,000
Convertible Preferred Stock
Current liabilities:
Redeemable convertible preferred stock, $0.00001 par value – 0 and 21,759,654 shares authorized at December 31, 2015 and 2014, respectively; 0 and 18,458,289 shares issued and outstanding at December 31, 2015 and 2014		$ 474,000

BALANCE SHEETS (Parenthetical)

BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2015	Dec. 31, 2014
Redeemable convertible preferred stock, shares authorized		22,004,436
Redeemable convertible preferred stock, shares issued		18,703,071
Redeemable convertible preferred stock, shares outstanding	0	18,703,071
Preferred stock, par value	$ 0.00001	$ 0.00001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	75,000,000	28,572,789
Common stock, shares issued	36,033,209	996,520
Common stock, shares outstanding	36,033,209	996,520
Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.00001	$ 0.00001
Redeemable convertible preferred stock, shares authorized	0	21,759,654
Redeemable convertible preferred stock, shares issued	0	18,458,289
Redeemable convertible preferred stock, shares outstanding	0	18,458,289
Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.00001	$ 0.00001
Redeemable convertible preferred stock, shares authorized	0	244,782
Redeemable convertible preferred stock, shares issued	0	244,782
Redeemable convertible preferred stock, shares outstanding	0	244,782

STATEMENTS OF OPERATIONS AND CO

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]
Revenues:	$ 7,712	$ 5,077	$ 888
Operating expenses:
Research and development	28,357	28,302	10,890
General and administrative	12,558	6,540	4,954
Total operating expenses	40,915	34,842	15,844
Loss from operations	(33,203)	(29,765)	(14,956)
Interest income	1,315	7	6
Interest expense	(1,732)	(487)	(254)
Other income (expense), net	(1,744)	(55)	71
Loss before provision for income taxes	(35,364)	(30,300)	(15,133)
Provision for income taxes	10	10	10
Net loss	(35,374)	(30,310)	(15,143)
Accretion to redemption value and cumulative dividends on preferred stock	(6,705)	(4,566)	(3,751)
Net loss attributable to common stockholders	$ (42,079)	$ (34,876)	$ (18,894)
Net loss per share attributable to common stockholders, basic and diluted	$ (4.90)	$ (35.25)	$ (24.46)
Shares used to compute net loss per share attributable to common stockholders, basic and diluted	8,595,247	989,453	772,320
Other comprehensive loss:
Changes in unrealized losses on short-term investments	$ (76)
Total other comprehensive loss	(76)
Comprehensive loss	$ (35,450)	$ (30,310)	$ (15,143)

STATEMENTS OF REDEEMABLE CONVER

STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Stockholder Notes	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit	Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred StockSeries B-1	Redeemable Convertible Preferred StockSeries C	Redeemable Convertible Preferred StockSeries D	Convertible Preferred Stock
Temporary equity, beginning balance at Dec. 31, 2012							$ 40,493				$ 474
Temporary equity, beginning balance, shares at Dec. 31, 2012							11,995,481				244,782
Beginning balance at Dec. 31, 2012	$ (26,045)		$ (393)			$ (25,652)
Beginning balance, shares at Dec. 31, 2012		779,989
Common stock issued in connection with a license agreement	199	$ 1		$ 198
Common stock issued in connection with a license agreement, shares		157,332
Exercise of stock options	$ 60			60
Exercise of stock options, shares	53,193	53,193
Interest on stockholder notes	$ (6)		(6)
Vesting of early exercise stock option	64			64
Stock-based compensation	343			343
Accretion to redemption value and cumulative dividends on preferred stock							$ 3,751
Accretion to redemption value and cumulative dividends on preferred stock	(3,751)			(665)		(3,086)
Net loss	(15,143)					(15,143)
Temporary equity, ending balance at Dec. 31, 2013							$ 44,244				$ 474
Temporary equity, ending balance, shares at Dec. 31, 2013							11,995,481				244,782
Ending balance at Dec. 31, 2013	(44,279)	$ 1	(399)			(43,881)
Ending balance, shares at Dec. 31, 2013		990,514
Issuance of preferred stock, net of issuance costs								$ 11,618	$ 15,808
Issuance of preferred stock, net of issuance costs, shares								3,355,107	3,107,701
Exercise of stock options	$ 8			8
Exercise of stock options, shares	6,006	6,006
Interest on stockholder notes	$ (5)		(5)
Vesting of early exercise stock option	58			58
Stock-based compensation	553			553
Accretion to redemption value and cumulative dividends on preferred stock							$ 4,566
Accretion to redemption value and cumulative dividends on preferred stock	(4,566)			(619)		(3,947)
Net loss	$ (30,310)					(30,310)
Temporary equity, ending balance at Dec. 31, 2014							$ 76,236				$ 474
Temporary equity, ending balance, shares at Dec. 31, 2014	18,703,071						18,458,289				244,782
Ending balance at Dec. 31, 2014	$ (78,541)	$ 1	(404)			(78,138)
Ending balance, shares at Dec. 31, 2014		996,520
Issuance of preferred stock, net of issuance costs									$ 4,969	$ 69,744
Issuance of preferred stock, net of issuance costs, shares									941,842	7,490,540
Issuance of preferred stock upon net exercise of warrants, shares								60,640
Conversion of redeemable convertible preferred stock to common stock in connection with initial public offering	159,163			159,163			$ (159,163)
Conversion of redeemable convertible preferred stock to common stock in connection with initial public offering, shares		26,951,311					(26,951,311)
Conversion of convertible preferred stock to common stock in connection with initial public offering	474			474							$ (474)
Conversion of convertible preferred stock to common stock in connection with initial public offering, shares		244,782									(244,782)
Issuance of common stock in connection with initial public offering, net of underwriting discount of $6,440 and offering costs of $3,796	81,764			81,764
Issuance of common stock in connection with initial public offering, net of underwriting discount of $6,440 and offering costs of $3,796, share		7,666,667
Extinguishment of preferred stock liability							$ 1,509
Extinguishment of preferred stock warrant liability	788			788
Exercise of stock options	$ 263			263
Exercise of stock options, shares	173,929	173,929
Interest on stockholder notes	$ (4)		(4)
Repayment on stockholders note	330		330
Stock-based compensation	3,986			3,986
Accretion to redemption value and cumulative dividends on preferred stock							$ 6,705
Accretion to redemption value and cumulative dividends on preferred stock	(6,705)			(2,751)		(3,954)
Other comprehensive loss	(76)				$ (76)
Net loss	$ (35,374)					(35,374)
Temporary equity, ending balance, shares at Dec. 31, 2015	0						0				0
Ending balance at Dec. 31, 2015	$ 126,068	$ 1	$ (78)	$ 243,687	$ (76)	$ (117,466)
Ending balance, shares at Dec. 31, 2015		36,033,209

STATEMENTS OF REDEEMABLE CONVE6

STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Offering cost for common stock issued in public offering	$ 3,796
Underwriting discount for common stock issued in public offering	6,440
Redeemable Convertible Preferred Stock | Series B-1
Issuance of temporary equity, adjustments to convertible preferred stock liability		$ 1,303
Temporary equity stock issued issuance costs		33
Redeemable Convertible Preferred Stock | Series C
Issuance of temporary equity, adjustments to convertible preferred stock liability		395
Temporary equity stock issued issuance costs	30	$ 298
Redeemable Convertible Preferred Stock | Series D
Temporary equity stock issued issuance costs	$ 255

STATEMENTS OF CASH FLOWS

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	$ (35,374)	$ (30,310)	$ (15,143)
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Loss on disposal of property and equipment	25		26
Depreciation and amortization	1,206	783	655
Amortization of debt discount	80	40	12
Accretion of discount on short-term investments	1,186
Issuance of common stock in connection with a license agreement			198
Stock-based compensation expense	3,986	553	343
Deferred income taxes	8	8	10
Changes in operating assets and liabilities
Accounts receivable	1,131	(1,638)	(237)
Prepaid expenses and other current assets	(1,491)	(261)	(88)
Other assets	128	(344)	(78)
Accounts payable	2,944	660	366
Accrued liabilities	3,170	793	495
Deferred revenue	(6,130)	61,463	5,500
Net cash (used in)/provided by operating activities	(27,415)	31,802	(8,008)
Cash flows from investing activities:
Purchases of property and equipment	(1,594)	(1,663)	(732)
Purchases of short-term investments	(250,901)
Maturities of short-term investments	122,750
Increase in restricted cash	(817)
Net cash used in investing activities	(130,562)	(1,663)	(732)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	74,430	26,802
Proceeds from exercise of stock options	263	8	62
Proceeds from initial public offering, net of issuance costs	81,777
Proceeds from issuance of notes payable			3,359
Repayment of notes payable	(3,067)	(1,256)	(724)
Net cash provided by financing activities	153,403	25,554	2,697
Net increase/(decrease) in cash and cash equivalents	(4,574)	55,693	(6,043)
Cash and cash equivalents, beginning of year	64,396	8,703	14,746
Cash and cash equivalents, end of year	59,822	64,396	8,703
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	100	68
Accretion to redemption value and cumulative dividends on preferred stock	6,705	4,566	3,751
Convertible preferred stock liability recorded in connection with redeemable convertible preferred stock	1,509	908
Stock issuance costs in accounts payable and accrued liabilities	13	284
Convertible preferred stock warrants issued in connection with debt			82
Issuance costs in accounts payable and accrued liabilities			198
Convertible Preferred Stock Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability	1,114	13	(110)
Convertible Preferred Stock Warrant Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability	$ 602	$ 42	$ 43

Description of the Business

Description of the Business	12 Months Ended
	Dec. 31, 2015
Disclosure Text Block [Abstract]
Description of the Business	1. Description of the Business CytomX Therapeutics, Inc. (the “Company”) is an oncology-focused biopharmaceutical company focused on developing Probody therapeutics for the treatment of cancer. Probody therapeutics are masked antibodies that remain inert in healthy tissue but are activated specifically in the disease microenvironment. The Company is located in South San Francisco, California and was incorporated in the state of Delaware in September 2010. Initial Public Offering On October 7, 2015, the Company’s registration statement on Form S-1 relating to its initial public offering (“IPO”) of its common stock was declared effective by the Securities and Exchange Commission (“SEC”) and the shares of its common stock began trading on The NASDAQ Global Select Market on October 8, 2015. The public offering price of the shares sold in the IPO was $12.00 per share. The IPO closed on October 14, 2015, pursuant to which the Company sold 7,666,667 shares of common stock, including the sale of 1,000,000 shares of common stock to the underwriters upon their exercise of their option to purchase additional shares. The Company received net proceeds of approximately $81.8 million, after underwriting discounts, commissions and estimated offering expenses. Immediately prior to the consummation of the IPO, all outstanding shares of convertible preferred stock and redeemable convertible preferred stock converted into common stock. Reverse Stock Split On October 2, 2015, the Company effected a one-for-62.997 reverse stock split of the Company’s issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock. The par values of the common stock, redeemable convertible preferred stock and convertible preferred stock were not adjusted as a result of the reverse stock split. All authorized and issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock and per share amounts contained in the accompanying financial statements have been retroactively adjusted to reflect this reverse stock split for all periods presented.

Liquidity

Liquidity	12 Months Ended
	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Liquidity	2. Liquidity Since inception, the Company has incurred recurring net operating losses. As December 31, 2015 and 2014, the Company had an accumulated deficit of $117.5 million and $78.1 million, respectively, and expects to incur losses for the next several years. Since its inception, the Company has funded its operations primarily through sales of its common stock in conjunction with the IPO, convertible preferred securities and payments received under its collaboration agreements. As of December 31, 2015, the Company had cash, cash equivalents and short-term investments of $186.7 million. In May and June 2015, the Company received aggregate net proceeds of $73.2 million from the issuance of its Series C and Series D redeemable convertible preferred stock. In October 2015, the Company consummated its IPO and raised net proceeds of approximately $81.8 million, after deducting underwriting discounts and commissions and offering expenses.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	3. Summary of Significant Accounting Policies Basis of Presentation The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The Company’s functional and reporting currency is the U.S. dollar. Use of Estimates The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates. Concentration of Credit Risk and Other Risks and Uncertainties The Company is subject to a number of risks similar to other biopharmaceutical companies in the early stage, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s products, and protection of proprietary technology. If the Company does not successfully obtain regulatory approval, commercialize or partner any of its product candidates, it will be unable to generate revenue from product sales or achieve profitability. Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents, short term investments and accounts receivable. Substantially all the Company’s cash is held by one financial institution that management believes is of high credit quality. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents in highly rated money market funds and its short-term investments in U.S. Government Bonds. Customers who represent 10% or more of the Company’s total revenue during each period presented or net accounts receivable balance at each respective balance sheet date are as follows: Revenue Accounts Receivable, net For December 31, 2015 2014 2013 2015 2014 Customer A 77 % 54 % * 50 % * Customer B 23 % 46 % 100 % 50 % 92 % * Less than 10%. All of the Company’s customers are located in the United States of America. Deferred Offering Costs Deferred offering costs, which consisted primarily of direct incremental costs related to the Company’s initial public offering of its common stock, were capitalized in other assets until the consummation of the initial public offering. These offering costs were reclassified to additional paid-in capital upon the closing of the initial public offering in October 2015. There were no deferred offering costs capitalized as of December 31, 2014. Segments Management has determined that it has one business activity and operates as one operating segment as it only reports financial information on an aggregate basis to its chief executive officer, who is the Company’s chief operating decision maker. All long-lived assets are maintained in the United States of America. Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents. Restricted Cash Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015. Restricted cash in 2014 represents amounts related to the security deposit for the Company’s credit card accounts. Short-term Investments All investments have been classified as “available-for-sale” and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities greater than 12 months at the date of purchase are considered long-term investments. Unrealized gains and losses, deemed temporary in nature, are reported as a component of accumulated other comprehensive income (loss), net of tax. A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the security. Premiums (discounts) are amortized (accreted) over the life of the related security as an adjustment to yield using the straight-line interest method. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold. Property and Equipment, net Property and equipment are recorded at cost net of accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the respective assets. The useful lives of property and equipment are as follows: Machinery and equipment 5 years Computer equipment and software 3 years Furniture and fixtures 3 years Leasehold improvements Shorter of remaining lease term or estimated life of the assets Maintenance and repairs that do not extend the life or improve the asset are expensed when incurred. Goodwill and Intangible Assets Goodwill represents the excess of the purchase price paid over the fair value of tangible and identifiable intangible assets acquired in business combinations. Goodwill and other intangible assets with indefinite lives are not amortized, but are assigned to reporting units and tested for impairment annually, or whenever there is an impairment indicator. Intangible assets are comprised of in-process research and development (“IPR&D”). The Company assesses impairment indicators annually or more frequently, if a change in circumstances or the occurrence of events suggests the remaining value may not be recoverable. Intangible assets that are not deemed to have an indefinite life are amortized over their estimated useful lives. There was no impairment of goodwill or intangible assets identified during the years ended December 31, 2015 and 2014. Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset (or asset group) may not be recoverable and prior to any goodwill impairment test. An impairment loss is recognized when the total of estimated undiscounted future cash flows expected to result from the use of the asset (or asset group) and its eventual disposition is less than its carrying amount. Impairment, if any, would be assessed using discounted cash flows or other appropriate measures of fair value. There was no impairment of long-lived assets during the periods presented in these financial statements. Convertible Preferred Stock Warrant Liability Freestanding warrants for shares that are contingently redeemable are classified as liabilities on the balance sheet at their estimated fair value because the shares underlying the warrants may obligate the Company to transfer assets to the holders at a future date under certain circumstances such as a deemed liquidation event. The warrants are subject to re-measurement at each balance sheet date and the change in fair value, if any, is included in other income (expense), net. The Company adjusted the liability for changes in fair value until the consummation of its IPO in October 2015, at which time all convertible preferred stock warrants were net exercised into shares of common stock and the related convertible preferred stock warrant liability was reclassified to additional paid-in capital. Convertible Preferred Stock Liability The obligation to issue additional shares of Series B-1 and Series C redeemable convertible preferred stock at a future date was determined to be a freestanding instrument that should be accounted for as a liability. At initial recognition, the Company recorded the convertible preferred stock liability on the balance sheets at its estimated fair value. The liability is subject to remeasurement at each balance sheet date, with changes in fair value recognized as a component of other income (expense), net. At the time of each funding, the Company remeasured the liability, with the change in fair value recognized as a component of other income (expense), net and then reclassified the fair value associated with the convertible preferred stock liability to the applicable series of redeemable convertible preferred stock. Immediately prior to the consummation of the Company’s IPO in October 2015, the convertible preferred stock converted to 27,135,453 shares of common stock. Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity (deficit) except those resulting from distributions to stockholders. The Company’s unrealized losses on short-term investments represent the only component of other comprehensive loss that is excluded from the reported net loss. Revenue Recognition The Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; transfer of technology has been completed or services have been rendered; the price to the customer is fixed or determinable and collectability is reasonably assured. The Company’s revenues are primarily derived through its license, research, development and commercialization agreements. The terms of these types of agreements may include (i) licenses to the Company’s technology, (ii) research and development services, and (ii) services or obligations in connection with participation in research or steering committees. Payments to the Company under these arrangements typically include one or more of the following: nonrefundable upfront and license fees, research funding, milestone and other contingent payments to the Company for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products. In arrangements involving the delivery of more than one element, each required deliverable is evaluated to determine whether it qualifies as a separate unit of accounting. The determination is based on whether the deliverable has “standalone value” to the customer. If a deliverable does not qualify as a separate unit of accounting, it is combined with the other applicable undelivered item(s) within the arrangement and these combined deliverables are treated as a single unit of accounting. The arrangement’s consideration that is fixed or determinable is allocated to each separate unit of accounting based on the relative selling price methodology in accordance with the selling price hierarchy, which includes vendor-specific objective evidence (“VSOE”) of selling price, if available, or third-party evidence of selling price if VSOE is not available, or the best estimate of selling price, if neither VSOE nor third-party evidence is available. Payments or reimbursements for the Company’s research and development efforts for the arrangements where such efforts are considered as deliverables are recognized as the services are performed and are presented on a gross basis. When upfront payments are received and if there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, the Company recognizes revenue ratably over the associated period of performance. The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones and sales-based milestones. Such payments are typically payable under the collaborations when the collaboration partner claims or selects a target, or initiates or advances a covered product candidate in preclinical or clinical development, upon submission for marketing approval of a covered product with regulatory authorities, upon receipt of actual marketing approvals of a covered product or for additional indications, or upon the first commercial sale of a covered product. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. Each contingent and milestone payment is evaluated to determine whether it is substantive and at risk to both parties. The Company recognizes any payment that is contingent upon the achievement of a substantive milestone entirely in the period in which the milestone is achieved. Any payments that are contingent upon achievement of a non-substantive milestone are recognized as revenue prospectively, when such payments become due and collectible, over the remaining expected performance period under the arrangement, which is generally the remaining period over which the research and development services are expected to be provided. Research and Development Expenses Research and development expenses include costs directly attributable to the conduct of research and development programs, including the cost of salaries, payroll taxes, employee benefits, materials, supplies, depreciation on and maintenance of research equipment, the cost of services provided by outside contractors, and the allocated portions of facility costs, such as rent, utilities, insurance, repairs and maintenance, depreciation, and general support services. All costs associated with research and development are expensed as incurred. Stock-Based Compensation The Company measures its stock-based awards made to employees based on the fair values of the awards as of the grant date using the Black-Scholes option-pricing model. Stock-based compensation expense is recognized over the requisite service period using the straight-line method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. As such, the Company’s stock-based compensation is reduced for the estimated forfeitures at the date of grant and revised in subsequent periods if actual forfeitures differ from those estimates. Stock-based compensation expense for options granted to non-employees as consideration for services received is measured on the date of performance at the fair value of the consideration received or the fair value of the equity instruments issued, using the Black-Scholes option-pricing model, whichever can be more reliably measured. Compensation expense for options granted to non- employees is periodically remeasured as the underlying options vest. Income Taxes The Company accounts for income taxes under the liability method which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses and research and development credit carryforwards. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized. The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. Net Loss per Share Attributable to Common Stockholders Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration of potentially dilutive securities. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of potentially dilutive securities is anti-dilutive. Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU 2014-09, Revenue from Contracts with Customers, which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The ASU will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. The new standard will be effective for the Company on January 1, 2018, which is the effective date for public companies. Early application is permitted as of January 1, 2017. The standard permits the use of either the retrospective or cumulative effect transition method. The Company is evaluating the effect that ASU 2014-09 will have on its financial statements and related disclosures. The Company has not yet selected a transition method nor has it determined the effect of the standard on its ongoing financial reporting. In August 2014, the FASB issued Accounting Standards Update No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern. In 2015, the FASB issued new guidance related to balance sheet classification of deferred taxes. The new guidance requires that deferred tax assets and liabilities be classified as noncurrent in a classified statement of financial position. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. Early adoption is permitted. As the Company’s deferred tax balance is already classified as noncurrent, this new guidance had no financial statement impact during the three months ended December 31, 2015. In February of 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)” (“ASU 2016-02”). Under ASU 2016-2, an entity will be required to recognize right-of-use assets and lease liabilities on its balance sheet and disclose key information about leasing arrangements. ASU 2016-02 offers specific accounting guidance for a lessee, a lessor and sale and leaseback transactions. Lessees and lessors are required to disclose qualitative and quantitative information about leasing arrangements to enable a user of the financial statements to assess the amount, timing and uncertainty of cash flows arising from leases. For public companies, ASU 2016-02 is effective for annual reporting periods beginning after December 15, 2018, including interim periods within that reporting period, and requires a modified retrospective adoption, with early adoption permitted. The Company plans to adopt this guidance beginning with its first quarter ending March 31, 2019. The Company is in the process of evaluating the future impact of ASU 2016-02 on its financial position, results of operations and cash flows.

Fair Value Measurements and Sho

Fair Value Measurements and Short-term Investments	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Disclosures	4. Fair Value Measurements and Short-Term Investments The Company records its financial assets and liabilities at fair value. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value, and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring fair value as follows: · Level I: Inputs which include quoted prices in active markets for identical assets and liabilities. · Level II: Inputs other than Level I that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. · Level III: Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. The carrying amounts of the Company’s financial instruments, including restricted cash, accounts receivable, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. Based on the borrowing rates available to the Company for debt with similar terms and consideration of default and credit risk using Level II inputs, the carrying value of the Company’s long-term debt as of December 31, 2014 approximates its fair value. The Company’s financial instruments consist of Level I and II assets and Level III liabilities. Level I assets consist primarily of highly liquid money market funds that are included in restricted cash. The Company’s Level II assets consist of U.S. government bonds that are included in short-term investments. The Company’s Level III liabilities include the convertible preferred stock warrant liability and the convertible preferred stock liability. The determination of the fair value of the convertible preferred stock warrant liability is discussed in Note 10. The determination of the fair value of the convertible preferred stock liability is discussed in Note 12. The following tables set forth the fair value of the Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands): December 31, 2015 Level I Level II Level III Total Assets Money market funds $ 44,714 $ — $ — $ 44,714 Restricted cash (money market funds) 917 — — 917 U.S. Government bonds — 140,392 — 140,392 $ 45,631 $ 140,392 $ — $ 186,023 December 31, 2014 Level I Level II Level III Total Assets Restricted cash (money market funds) $ 100 $ — $ — $ 100 $ 100 $ — $ — $ 100 Liabilities Convertible preferred stock warrant liability $ — $ — $ 186 $ 186 Convertible preferred stock liability — — 395 395 $ — $ — $ 581 $ 581 The following table sets forth the changes in the fair value of Level III liabilities (in thousands): Convertible Preferred Stock Warrant Liability Convertible Preferred Stock Liability Fair value at December 31, 2014 $ 186 $ 395 Change in fair value 602 1,114 Recognition of fair value upon issuance of redeemable convertible preferred stock (788 ) (1,509 ) Fair value at December 31, 2015 $ — $ — The following is a summary of the gross unrealized gains on the Company’s short-term investments (in thousands): December 31, 2015 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Investment Securities U.S. Government bonds $ 126,965 $ — $ (76 ) $ 126,889 Total securities $ 126,965 $ — $ (76 ) $ 126,889 The contractual maturities of securities classified as available-for-sale as of December 31, 2015 were as follows (in thousands): December 31, 2015 Due within one year $ 126,889 Total $ 126,889

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Property and Equipment	5. Property and Equipment Property and equipment, net consisted of the following (in thousands): December 31 2015 2014 Machinery and equipment $ 4,910 $ 4,059 Computer equipment and software 452 315 Furniture and fixtures 51 54 Leasehold improvements 720 183 Construction in progress 169 399 6,302 5,010 Less: accumulated depreciation and amortization (2,821 ) (1,992 ) $ 3,481 $ 3,018 Depreciation and amortization expense was $1.2 million, $783,000 and $655,000 for the years ended December 31, 2015, 2014 and 2013, respectively.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	12 Months Ended
	Dec. 31, 2015
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	6. Goodwill and Intangible Assets Goodwill and in-process research and development assets result from a series of integrated financing transactions in 2010 that was accounted for as a business combination. The in-process research and development relates to the Company’s proprietary Probody technology platform and is accounted for as an indefinite-lived intangible asset until the underlying project is completed or abandoned. Goodwill and intangible assets consisted of the following (in thousands): December 31, 2015 2014 Goodwill $ 949 $ 949 In-process research and development 1,750 1,750

Accrued Liabilities

Accrued Liabilities	12 Months Ended
	Dec. 31, 2015
Accrued Liabilities Current [Abstract]
Accrued Liabilities	7. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): December 31, 2015 2014 Payroll and related expenses $ 2,839 $ 859 Research and clinical expenses 1,562 276 Legal and professional expenses 296 418 Other accrued expenses 215 142 Total $ 4,912 $ 1,695

Research and Collaboration Agre

Research and Collaboration Agreements	12 Months Ended
	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Research and Collaboration Agreements	8. Research and Collaboration Agreements Pfizer Inc. In May 2013, the Company and Pfizer Inc. (“Pfizer”) entered into a Research Collaboration, Option and License Agreement (the “Pfizer Agreement”) to collaborate on the discovery and preclinical research activities related to Probody therapeutics, and Probody drug conjugates (“PDCs”) for research project targets nominated by Pfizer. Pfizer nominated two research targets in 2013 and had the option of nominating two additional research targets. In December 2014, Pfizer selected an additional research target. The Pfizer Agreement provides Pfizer with an option to acquire an exclusive development and commercialization license for each research project target. Upon exercise of the option, Pfizer (1) will receive an exclusive development and commercialization license for use of the Probody therapeutic during the development, manufacturing and commercialization of the potential product, and (2) will be responsible for the development, manufacturing and commercialization of such potential products. Pursuant to the Pfizer Agreement, the Company received an upfront payment of $6 million and is entitled to contingent payments of up to an aggregate of $626.5 million as follows: (i) $1.5 million for each of the two additional targets; (ii) up to $12.0 million upon exercise of the license options, (iii) up to $25.0 million from the achievement of development milestones for each research target program, or up to $82.0 million if the maximum of four research targets are selected by Pfizer; and (iv) up to $98.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program or up to $249.5 million if the maximum of four research targets are selected and (v) up to $100.0 million in sales milestones payments per research target program, or up to $280.0 million if the maximum of four research targets are selected by Pfizer. The Company is entitled to receive royalties in the mid-single digits to low teens on initial targets and mid-single digit royalties on additional targets from potential future sales of product candidates. The Company will also receive research and development service fees based on a prescribed full-time employee (“FTE”) rate per year that is capped. In accordance with ASC 605-25, the Company identified the following deliverables at the inception of the Pfizer Agreement: (1) the research license, (2) the research services and (3) the obligation to participate in the joint research committee. The Company determined that the research license does not have stand-alone value to Pfizer due to specialized nature of the research services to be provided by the Company, and accordingly, this deliverable was combined with the research services and participation in the joint research committee as a single unit of accounting. The Company concluded that, at the inception of the agreement, Pfizer’s options to obtain an exclusive development and commercialization license for each research project target do not represent deliverables of the agreement because they are substantive options and do not contain a significant or incremental discount. The upfront payment of $6.0 million was recorded as deferred revenue and is being recognized on a ratable basis over the estimated performance period of seven years. In December 2014, Pfizer selected an additional target and paid $1.5 million, which was recorded as deferred revenue and will be recognized over the remaining performance period. During the years ended December 31, 2015, 2014 and 2013, the Company recognized revenue of $1.8 million, $2.3 million and $0.9 million, respectively. As of December 31, 2015 and 2014, deferred revenue relating to the Pfizer Agreement was $4.9 million and $6.1 million, respectively. The amount due from Pfizer under the Agreement was $0.4 million and $1.7 million as of December 31, 2015 and 2014, respectively. ImmunoGen, Inc. In January 2014, the Company and ImmunoGen, Inc. (“ImmunoGen”) entered into the Research Collaboration Agreement (the “ImmunoGen Agreement”). The ImmunoGen Agreement provides the Company with the right to use ImmunoGen’s Antibody Drug Conjugate (“ADC”) technology in combination with the Company’s Probody technology to create Probody Drug Conjugates (“PDC”) directed at one specified target under a research license, and to subsequently obtain an exclusive, worldwide development and commercialization license to use ImmunoGen’s ADC technology to develop and commercialize such PDCs. The Company made no upfront cash payment in connection with the execution of the agreement. Instead, the Company provided ImmunoGen with the rights to CytomX’s Probody technology to create PDCs directed at two targets under the research license and to subsequently obtain exclusive, worldwide development and commercialization licenses to develop and commercialize such PDCs. Under the research licenses, the parties have one replacement right for each target, which needs to be made before the third anniversary of the agreement execution. Under the terms of the agreement, both the Company and ImmunoGen are required to perform research activities on behalf of the other party for no monetary consideration. The research activities for a particular target will last until January 2018 unless they are terminated by one of the parties or when a development and commercialization license is obtained with respect to that target. Each party is solely responsible for the development, manufacturing and commercialization of any products resulting from the exclusive development and commercialization license obtained by such party under the agreement. Each party may be liable to pay annual maintenance fees to the other party if the licensed product candidate covered under each development and commercialization license has not progressed to the clinical stage of development within six years of the exercise of the development and commercialization license. In consideration for the exclusive development and commercialization license that may be obtained by ImmunoGen, the Company is entitled to receive up to $30.0 million in development and regulatory milestone payments per the research program target, up to $50.0 million in sales milestone payments per target and royalties in the mid-single digits on the commercial sales of any resulting product. For the development and commercialization license that may be obtained by the Company, ImmunoGen is entitled to receive up to $60.0 million in development and regulatory milestone payments, up to $100.0 million in sales milestone payments and royalties in the mid to high single digits on the commercial sales of any resulting product. The Company accounted for the ImmunoGen Agreement based on the fair value of the assets and services exchanged. The Company identified the following significant deliverables at the inception of the ImmunoGen Agreement: (1) the research license, (2) the research services, (3) the obligation to participate in the joint research committee, (4) the exclusive research, development and commercialization license and (5) the obligation to provide future technology improvements, when available. The Company determined that the research license, participation in the joint steering committee and the research services do not have stand-alone value from the development and commercialization license and therefore those deliverables were combined into one unit of accounting. The Company considered factors such the limited economic benefits to ImmunoGen if development and commercialization license is not obtained and the lack of sublicensing rights in the research license. The estimated total fair value of the consideration of $13.2 million was recorded as deferred revenue, of which $13.0 million was allocated to the unit of accounting comprised of the research license, research services, participation in the joint research committee and the development and commercialization license, and $0.2 million was allocated to the future technological improvements. The Company will recognize $13.0 million upon delivery of development and commercialization licenses and will recognize amount allocated to the future technology improvements over the term of the license. The estimated fair value of assets and services received was also $13.2 million, of which $12.7 million was allocated to the licenses received and was charged to research and development expense, with the remaining amount of $0.5 million was allocated to the research services, joint research committee participation and technology improvements, which will be expensed over the period of services to be provided. Bristol-Myers Squibb Company On May 23, 2014, the Company and Bristol-Myers Squibb Company (“BMS”) entered into a Collaboration and License Agreement (the “BMS Agreement”) to discover and develop compounds for use in human therapeutics aimed at multiple immuno- oncology targets using the Company’s Probody technology. The effective date of the BMS Agreement was July 7, 2014. Under the terms of the BMS Agreement, the Company granted BMS exclusive worldwide rights to develop and commercialize Probody therapeutics for up to four oncology targets. BMS will have additional rights to substitute up to two collaboration targets. Each collaboration target has a two year research term and the two additional targets must be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target can be extended in one year increments up to three times. Pursuant to the BMS Agreement, the financial consideration from BMS was comprised of an upfront payment of $50.0 million and contingent payments of up to an aggregate of $1,217.0 million as follows: (i) up to $25.0 million for additional targets; (ii) up to $114.0 million in development milestone payments per research target program or up to $456.0 million if the maximum of four research targets are selected; (iii) up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program or up to $496.0 million if the maximum of four research targets are selected, and (iv) up to $60.0 million in sales milestones payments per research target program or up to $240.0 million if maximum of four research targets are selected. The Company is entitled to royalty payments in the mid to high single digits to low teens from potential future sales. The Company will also receive research and development service fees based on a prescribed FTE rate that is capped. The BMS Agreement also provided the Company to sell to BMS the Company’s common stock upon an IPO. In connection with the IPO in October 2015, BMS purchased 833,333 shares of the Company’s common stock at the initial public offering price and on the same terms as other purchasers in the offering. The Company identified the following deliverables at the inception of the BMS Agreement: (1) the exclusive research, development and commercialization license (“license”), (2) the research and development services and (3) the obligation to participate in the joint research committee. The Company determined that the license does not have stand-alone value to BMS without the Company’s research services and expertise related to the development of the product candidates, and accordingly, it was combined with the research services and participation in the joint research committee as a single unit of accounting. The Company received an upfront payment of $50.0 million from BMS in July 2014. The upfront payment was recorded as deferred revenue and being recognized on a ratable basis over the estimated performance period of ten years. The Company determined that the remaining contingent payments under the Agreement do not constitute substantive milestones and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments do not meet the definition of a substantive milestone because the achievement of these events solely depends on BMS’s performance. Accordingly, any revenue from these contingent payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligations, if any, relating to this arrangement. If there are no remaining performance obligations under the arrangement at the time the contingent payment is triggered, the contingent payment will be recognized as revenue in full upon triggering the event. During the years ended December 31, 2015 and 2014, the Company recognized revenue of $5.9 million and $2.8 million, respectively, under the BMS Agreement. As of December 31, 2015 and 2014, deferred revenue relating to the BMS Agreement was $42.6 million and $47.6 million, respectively. The amount due from BMS under the BMS Agreement was $0.4 million and $0.1 million as of December 31, 2015 and 2014, respectively. MD Anderson In November 2015, the Company entered into a research collaboration agreement with MD Anderson to research Probody-enabled chimeric antigen receptor killer (CAR-NK) cell therapies, known as ProCAR-NK cell therapies. Under this collaboration, MD Anderson will use the Company’s Probody technology to conduct research of ProCAR-NK cell therapies against certain targets selected by the Company in cancer immunotherapy. Under the research collaboration agreement, the Company has the right to exercise an option, during the option period expiring on November 2, 2019 and upon payment of an option exercise fee, to negotiate and acquire a worldwide, exclusive, sublicensable license from MD Anderson for development and commercialization of products directed against any of the selected targets. The research collaboration agreement will continue in effect until the earlier of (i) the date that the Company exercises the option to acquire the license from MD Anderson and (ii) the expiration of the option period. The impact of this agreement was not material for the financial statements for the year ended December 31, 2015.

License Agreement

License Agreement	12 Months Ended
	Dec. 31, 2015
Research And Development [Abstract]
License Agreement	9. License Agreement The Company has an exclusive, worldwide license agreement (the “UC Agreement”) with the Regents of the University of California (the “UC Regents”), acting through its Santa Barbara Campus, relating to the use of certain patents and technology relating to its core technology, including its therapeutic antibodies. Pursuant to the UC Agreement, the Company is obligated to (i) make royalty payments to the UC Regents on net sales of its products covered under the agreement, subject to annual minimum amounts,(ii) make milestone payments to the UC Regents upon the occurrence of certain events, (iii) make a milestone payment to the UC Regents upon occurrence of an IPO or change of control, and (iv) reimburse the UC Regents for prosecution and maintenance of the licensed patents. If the Company sublicenses its rights under the UC Agreement, it is obligated to pay the UC Regents a percentage of the total gross proceeds received in consideration of the grant of the sublicense, which total amount would be first reduced by the aggregate amount of certain research and development related expenses incurred by the Company. In 2013, the Company amended the UC Agreement to reduce the amounts due the UC Regents upon receipt by the Company of upfront payments, milestone payments and royalties from sublicensees. In exchange for this amendment, the Company issued to the UC Regents 157,332 shares of common stock. The UC Agreement, as amended, will remain in effect until the expiration or abandonment of the last to expire of the licensed patents. In the years ended December 31, 2015, 2014 and 2013, the Company incurred expenses of $347,000, $657,000 and $714,000 respectively, to the UC Regents under the provisions of the agreement. Royalty obligations The Company has future minimum royalty obligations due under the terms of certain exclusive licensed patent rights. These minimum future obligations are as follows (in thousands): Year ended December 31, 2016 $ 150 2017 150 Total minimum royalty obligations $ 300

Long-term Debt

Long-term Debt	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
Long-term Debt	10. Long-term Debt In May 2012, the Company entered into a Master Loan and Security Agreement (the “Debt Facility”). Under the terms of the agreement, an aggregate of $2.0 million could be drawn down during the initial basic loan term of 42 months. In January and December 2013, the Company amended the Debt Facility to borrow an additional $0.3 million and $3.0 million, respectively, with similar terms. Borrowings under the debt facility bear interest at 11.74% per annum. The Company’s obligations under the Debt Facility are collateralized by a security interest in substantially all of its assets, excluding its intellectual property and certain other assets. The Debt Facility also contains customary conditions related to borrowing, events of default, and covenants, including covenants limiting the Company’s ability to dispose of assets, undergo a change in control, merge with or acquire other entities, incur debt, incur liens, pay dividends or other distributions to holders of its capital stock, repurchase stock and make investments, in each case subject to certain exceptions. The agreement also allows the lender to call the debt in the event there is a material adverse change in the Company’s business or financial condition. In connection with the execution and the amendment of the Debt Facility, the Company issued warrants to the lender to purchase an aggregate of 81,620 shares of the Company’s Series B-1 redeemable convertible preferred stock. The warrants expire at the earlier of (i) the tenth anniversary of issuance, (ii) upon the consummation of an IPO of the Company’s common stock, or (iii) the consummation of certain change of control events. The warrants are exercisable in cash at an exercise price of $3.084396 per share or through a cashless exercise provision. Under the cashless exercise provision, the holder may, in lieu of payment of the exercise price in cash, surrender the warrant and receive a net amount of shares based on the fair market value of the Company’s Series B-1 redeemable convertible preferred stock at the time of exercise of the warrant after deducting the aggregate exercise price. If the warrant has not been previously exercised, the cashless exercise provision is automatically triggered upon expiration if the fair value of the Series B-1 redeemable convertible preferred stock is higher than the exercise price of the warrants. In the event that all of the Company’s Series B-1 redeemable convertible preferred stock have been converted into common stock, the warrants will be exercisable for the same number of shares of common stock at the same exercise price. In connection with the consummation of the IPO in October 2015, all of the warrants were net exercised, resulting in issuance of an aggregate of 60,640 shares of our common stock. Upon issuance of the warrants, the Company recorded a preferred stock warrant liability based on its initial fair value estimated using the Black-Scholes model with an offset to debt discount. The debt discount is amortized to interest expense using the effective interest method over the term of the Debt Facility. The warrant liability is subject to remeasurement to fair value at each balance sheet date until the earliest of the exercise or expiration of the convertible preferred stock warrant, and any change in fair value is recognized in other income (expense), net. The Company repaid and terminated the Debt Facility in September 2015.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Operating Lease New Lease Agreement On December 10, 2015, the Company entered into a lease (the “New Lease”) with HCP Oyster Point III LLC (the “Landlord”) to lease approximately 76,173 rentable square feet of office and laboratory space located in South San Francisco, California for the Company’s new corporate headquarters. The Company currently leases office and laboratory space located in South San Francisco, California, pursuant to a lease agreement which expires in 2019. On March 1, 2016, the Company entered into an agreement to terminate the current lease, which provides for early termination effective November 30, 2016 – see Note 21 for more information. The term of the New Lease commences on the later of (i) the date that the Landlord’s construction and tenant improvements have been completed pursuant to the New Lease and (ii) October 1, 2016. The New Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the New Lease. The New Lease provides for annual base rent of approximately $3.1 million in the first year of the lease term, which will increase on an annual basis beginning from the 25 th In addition, the Company obtained a standby letter of credit (the “Letter of Credit”) in an amount of approximately $0.9 million, which may be drawn by the Landlord to be applied for certain purposes upon the Company’s breach of any provisions under the New Lease. The Company has recorded the $0.9 million Letter of Credit in restricted cash, non-current within its balance sheet at December 31, 2015. Rent expense is recognized on a straight-line basis over the term of the lease and accordingly the Company records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. The future minimum lease payments for all of the Company’s facility leases are as follows (in thousands): Year Ending December 31: 2016 $ 1,369 2017 4,251 2018 5,268 2019 4,582 2020 and beyond 34,144 Total $ 49,614 Rent expense during the years ended December 31, 2015, 2014 and 2013 was $940,000, $836,000 and $529,000, respectively. Legal Proceedings The Company is subject to claims and assessments from time to time in the ordinary course of business but do not believe that any such matters, individually or in the aggregate, will have a material adverse effect on the Company’s financial position, results of operations or cash flows. Indemnifications In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently has directors’ and officers’ insurance.

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Convertible Preferred Stock	12. Convertible Preferred Stock In December 2014, the Company granted a second tranche option (“Second Tranche Option”) to one of its investors to purchase 659,209 shares of its Series C redeemable convertible preferred stock upon the achievement of certain milestones. At initial recognition, the Company recorded the Second Tranche Option as a derivative liability on the balance sheet at its estimated fair value of $395,000. In May 2015, the Company achieved the relevant milestones and the investor exercised their right to purchase 659,209 shares of Series C convertible redeemable preferred stock for net proceeds of $3.5 million. Immediately prior to the closing of this tranche, the Company remeasured the preferred stock liability to its then fair value and recorded a loss from remeasurement of $1.1 million in other income (expense), net. The fair value of the preferred stock liability in the amount of $1.5 million was reclassified to redeemable convertible preferred stock. As of December 31, 2014, the outstanding convertible preferred stock was as follows (in thousands, except share amounts): December 31, 2014 Shares Authorized Shares Issued and Outstanding Net Carrying Value Series A-1 33,101 33,101 $ 49 Series A-2 211,681 211,681 425 Series B-1 14,944,578 14,488,176 57,695 Series B-2 862,412 862,412 2,698 Series C 5,952,664 3,107,701 15,843 Total 22,004,436 18,703,071 $ 76,710 In connection with the consummation of the IPO in October 2015, all outstanding shares of Series A-1, Series A-2, Series B-1, Series B-2, Series C and Series D were converted into 27,135,453 shares of common stock on a one-for-one basis. As such, no convertible preferred stock shares were outstanding as of December 31, 2015.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Common Stock	13. Common Stock In October 2015, the Company’s board of directors and stockholders approved the amended and restatement of the Company’s certificate of incorporation. The Amended and Restated Certificate of Incorporation was effective as of October 14, 2015, which provides for 75,000,000 authorized shares of common stock with par value of $0.00001 per share and 10,000,000 shares of preferred stock with a par value of $0.00001 per share. Common stockholders are entitled to dividends if and when declared by the Board of Directors subject to the prior rights of the preferred stockholders. As of December 31, 2015 and 2014, no dividends on common stock had been declared by the Board of Directors. The Company had reserved shares of common stock for issuance, on an as-converted basis, as follows: December 31, 2015 2014 Convertible preferred stock outstanding — 18,703,071 Options issued and outstanding 5,270,751 2,147,872 Convertible preferred stock warrants — 81,620 Shares available for future stock option grants 2,401,406 1,896,617 7,672,157 22,829,180

Stock Option Plans

Stock Option Plans	12 Months Ended
	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Option Plans	14. Stock Option Plans In 2010, the Company adopted its 2010 Stock Incentive Plan (the “2010 Plan”) which provided for the granting of stock options to employees, directors and consultants of the Company. Options granted under the 2010 Plan were either incentive stock options (“ISOs”) or nonqualified stock options (“NSOs”). In February 2012, the Company adopted its 2011 Stock Incentive Plan (the “2011 Plan”). The 2011 Plan is divided into two separate equity programs, an option and stock appreciation rights grant program and a stock award program. In conjunction with adopting the 2011 Plan, the Company discontinued the 2010 Plan and released the shares reserved and still available under that plan. In connection with the consummation of the IPO in October 2015, the board of directors adopted the Company’s 2015 Equity Incentive Plan (the “2015 Plan” and collectively with the 2010 Plan and 2011 Plan, the “Plans”). In conjunction with adopting the 2015 Plan, the Company discontinued the 2011 Plan with respect to new equity awards. Options under the 2015 Plan may be granted for periods of up to ten years. All options issued to date have had a 10-year life. Under the terms of the 2015 Plan, options may be granted at an exercise price not less than the estimated fair value of the shares on the date of grant, as determined by the Company’s board of directors. For employees holding more than 10% of the voting rights of all classes of stock, the exercise price of ISOs and NSOs may not be less than 110% of the estimated fair value of the shares on the date of grant, as determined by the board of directors. To date, options granted generally vest over four years and vest at a rate of 25% upon the first anniversary of the issuance date and 1/48th per month thereafter. Activity under the Company’s stock option plans is set forth below: Options Outstanding Options Available for Grant Number of Options Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Life (years) Aggregate Intrinsic Value (in thousands) Balances at December 31, 2012 1,579,860 940,262 $ 1.197 Options granted (810,390 ) 810,390 1.008 Options exercised — (53,193 ) 1.134 Options forfeited 7,158 (7,158 ) — Retirement of shares under the 2010 Plan — (3,811 ) 1.071 Balances at December 31, 2013 776,628 1,686,490 1.071 8.5 $ 291 Options authorized 1,587,377 — — Options granted (697,557 ) 697,557 1.449 Options exercised — (6,006 ) 1.260 Options forfeited 230,169 (230,169 ) 0.945 Balances at December 31, 2014 1,896,617 2,147,872 1.197 8.1 $ 767 Options authorized 3,801,597 — — Options granted (3,309,708 ) 3,309,708 5.174 Options exercised — (173,929 ) 1.507 Options forfeited 12,900 (12,900 ) 1.405 Balances at December 31, 2015 2,401,406 5,270,751 3.694 8.6 $ 90,542 Options Exercisable—December 31, 2015 1,908,638 1.789 7.2 $ 36,418 Options vested and expected to vest—December 31, 2015 5,206,151 3.685 8.6 $ 89,476 The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the underlying common stock as of December 31, 2015, 2014 and 2013, respectively. The aggregate intrinsic value of stock options exercised in the years ended December 31, 2015, 2014 and 2013 was $2.3 million, $1,500 and $600, respectively. The total fair value of options that vested in the years ended December 31, 2015, 2014 and 2013 were $2.8 million, $410,000 and $268,000, respectively. The following table summarizes information about stock options outstanding and exercisable by exercise price at December 31, 2015: Outstanding Exercisable Exercise Price Number Outstanding Weighted-Average Remaining Contractual Life (Years) Number Exercisable $0.945 411,576 7.16 278,861 $1.134 572,079 5.61 572,079 $1.260 342,793 7.05 306,083 $1.386 164,844 8.05 148,970 $1.449 249,886 8.32 183,100 $1.512 240,058 8.78 81,281 $1.575 815,030 9.13 160,073 $4.473 464,799 9.35 24,629 $6.615 1,966,357 9.63 126,667 $12.00 26,895 9.77 26,895 $21.51 16,434 9.95 — 5,270,751 1,908,638 The following table summarizes information about stock options outstanding and vested by exercise price at December 31, 2014: Outstanding Exercisable Exercise Price Number Outstanding Weighted-Average Remaining Contractual Life (Years) Number Exercisable $0.945 427,329 8.16 214,899 $1.134 661,891 6.59 564,812 $1.260 361,384 8.18 299,106 $1.386 178,072 9.05 81,099 $1.449 269,647 9.34 70,711 $1.512 249,549 9.78 10,582 2,147,872 1,241,209 The options granted in the years ended December 31, 2015, 2014 and 2013 had a weighted average per share grant-date fair value of $7.169, $0.945, and $0.504, respectively. At December 31, 2015, the unrecognized compensation expense with respect to options granted to employees was $20.6 million, and is expected to be recognized over 3.5 years, respectively. Early Exercise of Employee Options Certain stock options granted under the Plans provide option holders the right to elect to exercise unvested options in exchange for restricted common stock. Such unvested restricted shares are subject to a repurchase right held by the Company at the original issuance price in the event the optionee’s service to the Company is terminated either voluntarily or involuntarily. The right usually lapses 25% on the first anniversary of the vesting start date and in 36 equal monthly amounts thereafter. These repurchase terms are considered to be a forfeiture provision. The cash or full recourse notes received from employees for exercise of unvested options is treated as a refundable deposit and is classified as a liability on the balance sheets.

Stock Based Compensation

Stock Based Compensation	12 Months Ended
	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Based Compensation	15. Stock Based Compensation Total stock-based compensation recorded related to option granted to employees and nonemployees was as follows (in thousands): Year Ended December 31, 2015 2014 2013 Research and development $ 1,972 $ 195 $ 121 General and administrative 2,014 358 222 Total stock-based compensation expense $ 3,986 $ 553 $ 343 Stock based compensation expense for employees was $3.2 million, $459,000 and $317,000 for the years ended December 31, 2015, 2014 and 2013, respectively. Stock-based compensation expense related to stock options granted to non-employees is recognized as the stock options are earned. The Company determined that the estimated fair value of the stock options is more readily measurable than the fair value of the services received. The fair value of stock options granted to non-employees is calculated at each grant date and re-measured at each reporting date using the Black-Scholes option pricing model. The stock-based compensation expense related to a grant will fluctuate as the estimated fair value of the common stock fluctuates over the period from the grant date to the vesting date. Stock based compensation expense for non-employees was $773,000, $94,000 and $26,000 for the years ended December 31, 2015, 2014 and 2013, respectively. The Company estimated the fair value of employee stock options using the Black-Scholes valuation model. The fair value of employee stock options is being amortized on a straight-line basis over the requisite service period of the awards. The fair value of employee stock options was estimated using the following assumptions for the year ended December 31, 2015, 2014 and 2013: Year Ended December 31, 2015 2014 2013 Expected volatility 62.9% – 68.9% 66.4% – 71.2% 70.8% – 71.7% Risk-free interest rate 1.4% – 1.9% 1.6% – 2.0% 0.9% – 1.9% Dividend yield — % — % — % Expected term (in years) 5.2 – 7.2 5.3 – 6.1 5.5 – 6.1 Expected Term . The expected term of stock options represents the period that the stock options are expected to remain outstanding and is based on industry peers, as the Company did not have sufficient historical performance to develop reasonable expectations about future exercise patterns and post-vesting employment information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior. Expected Volatility . The expected stock price volatility for the Company’s stock options was determined by examining the historical volatilities for comparable publicly traded companies within the biotechnology and pharmaceutical industry using an average of historical volatilities of Company’s industry peers. Risk-Free Interest Rate . The risk-free interest rate is based on the U.S. Treasury whose term was consistent with expected term of the Company’s stock options. Dividend Rate . The expected dividend was assumed to be zero as the Company has never paid dividends and has no current plans to do so. Expected Forfeiture Rate . The forfeiture rates were estimated based on actual employee head count and were immaterial to the financial statements during 2015, 2104 and 2013.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2015
Related Party Transactions [Abstract]
Related Party Transactions Disclosure	16. Related Party Transactions Certain employees of Third Rock Ventures, a stockholder of the Company, provide consulting services to the Company. General and administrative expenses for these services were $33,000, $46,000, $17,000 for the year ended December 31, 2015, 2014 and 2013, respectively. The amounts outstanding and included in accounts payable were $0 and $17,600 as of December 31, 2015 and 2014, respectively. The Company entered into full recourse loans (“stockholder notes” or “loans”) with current and former executive officers. Principal and interest under these loans are due at the earliest of (i) the fifth anniversary of the related note, (ii) the sale of the shares securing the notes, or (iii) thirty days after the termination of services. The principal loan amount and the accrued interest are reported as a deduction from stockholders’ equity (deficit) on the Company’s balance sheets. Loans made to two of the Company’s current and former executive officers were repaid and terminated in August 2015. The remaining balance of these loans was approximately $78,000 and $404,000 at December 31, 2015 and 2014, respectively. Interest income of $4,000, $5,000 and $6,000 was recorded in the years ended December 31, 2015, 2014 and 2013, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	17. Income Taxes The Company derives its income only from the United States. The components of the provision (benefit) for income taxes are as follows (in thousands): Years Ended December 31, 2015 2014 2013 Current: Federal $ — $ — $ — State 2 1 1 Total current 2 1 1 Deferred: Federal 8 9 9 State — — — Total deferred 8 9 9 Provision for income taxes $ 10 $ 10 $ 10 A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows: Years Ended December 31, 2015 2014 2013 U.S. federal taxes at statutory rate 34.0 % 34.0 % 34.0 % State tax, net of federal benefit 0.8 % 1.0 % 6.6 % Stock compensation (1.1 )% (0.3 )% (0.5 )% Tax attributes subject to 382 limitation (35.4 )% 0.0 % 0.0 % Tax credits 0.8 % 1.3 % 3.3 % Other (1.8 )% (0.1 )% (0.9 )% Change in valuation allowance 2.7 % (35.9 )% (42.6 )% Total — % — % (0.1 )% The types of temporary differences that give rise to significant portions of the Company’s deferred income tax liabilities are set out below (in thousands): Year Ended December 31, 2015 2014 2013 Net operating loss carryforwards $ 5,688 $ 22,484 $ 13,900 Research and development credits 1,337 2,009 1,324 Intangible—in-process R&D 88 96 124 Deferred revenue 16,182 1,579 — Accruals and deferred rent 998 276 321 Stock-based compensation 1,125 155 86 Other 26 — — Total gross deferred income tax assets 25,444 26,599 15,755 Less: valuation allowance (25,043 ) (26,012 ) (15,130 ) Deferred tax assets, net of valuation allowance 401 587 625 Fixed assets (313 ) (491 ) (419 ) In-process R&D (595 ) (595 ) (697 ) Deferred tax liabilities (908 ) (1,086 ) (1,116 ) Net deferred income tax liabilities $ (507 ) $ (499 ) $ (491 ) A valuation allowance has been established for the portion of deferred assets for which realization is not probable. The net change in the total valuation allowance for the year ended December 31, 2015 was a decrease of $1.0 million and for the years ended December 31, 2014 The Company has net operating loss carryforwards for federal and state income tax purposes of approximately $14.3 million and $14.3 million, respectively, as of December 31, 2015 available to reduce future income subject to income taxes. The federal and state net operating loss carryforwards will begin to expire in 2031 if not utilized. The Company also has federal and state research and development tax credits carryforwards of $0.4 million and $2.1 million, respectively, as of December 31, 2015 available to reduce future income taxes. The federal research and development tax credits will begin to expire in 2030 if not utilized. The state research and development tax credits have no expiration date. Internal Revenue Code section 382 places a limitation (the “Section 382 Limitation”) on the amount of taxable income that can be offset by net operating loss (“NOL”) carryforwards after a change in control (generally greater than 50% change in ownership) of a loss corporation. California has similar rules. The Company has performed an IRC Section 382 analysis and determined there was an ownership change in 2015. As a result, the federal and state carryforwards associated with the NOL and credit deferred tax assets were reduced by the amount of tax attributes estimated to expire during their respective carryforward periods. There may be further ownership changes after December 31, 2015. The Company had approximately $0.7 million of unrecognized tax benefits as of December 31, 2015, none of which would affect the Company’s effective tax rate if recognized, due to the Company’s valuation allowance. A reconciliation of the beginning and ending unrecognized tax benefit amount is as follows (in thousands): Year Ended December 31, 2015 2014 2013 Balance at the beginning of the year $ 3,019 $ 532 $ 200 Additions based on tax positions related to current year (2,312 ) 2,473 60 Adjustment based on submitted prior year tax returns (41 ) 14 272 Balance at end of the year $ 666 $ 3,019 $ 532 The Company recognizes interest and penalties related to uncertain tax positions in income tax expense. To the extent accrued interest and penalties do not ultimately become payable, amounts accrued will be reduced and reflected as a reduction of the provision for income taxes in the period that such determination is made. Interest and penalties have not been accrued at December 31, 2015, 2014 and 2013. The Company files income tax returns in the United States, including California state jurisdiction. The tax years 2010 to 2015 remains open to U.S. federal and state examination to the extent of the utilization of net operating loss and credit carryovers. As of December 31, 20

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	18. Defined Contribution Plan The Company sponsors a defined contribution plan under Section 401(k) of the Internal Revenue Code covering substantially all full-time U.S. employees. Employee contributions are voluntary and are determined on an individual basis subject to the maximum allowable under federal tax regulations. During the years ended December 31, 2015, 2014 and 2013, the Company made contributions to the plan of $25,000, $16,500 and $9,000, respectively.

Net Loss Per Share Attributable

Net Loss Per Share Attributable to Common Stockholders	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	19. Net Loss Per Share Attributable to Common Stockholders The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented, because including them would have been anti-dilutive: Year Ended December 31, 2015 2014 2013 Redeemable convertible preferred stock (on an as-converted basis) 17,507,788 15,024,251 11,995,481 Convertible preferred stock (on an as-converted basis) 192,473 244,782 244,782 Options to purchase common stock 3,865,842 1,987,532 1,482,579 Convertible preferred stock warrants 64,178 81,620 36,559 Total 21,630,281 17,338,185 13,759,401 A reconciliation of the numerator and denominator used in the calculation of the basic and diluted net loss per share attributable to common stockholders is as follows (in thousands except share and per share amounts): Year Ended December 31, 2015 2014 2013 Numerator: Net loss $ (35,374 ) $ (30,310 ) $ (15,143 ) Add: accretion to redemption value and cumulative dividends on preferred stock (6,705 ) (4,566 ) (3,751 ) Net loss attributable to common stockholders (42,079 ) (34,876 ) (18,894 ) Denominator: Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted 8,595,247 989,453 772,320 Net loss per share attributable to common stockholders, basic and diluted $ (4.90 ) $ (35.25 ) $ (24.46 )

Supplementary Data - Quarterly

Supplementary Data - Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Supplementary Data – Quarterly Financial Data (Unaudited)	20. Supplementary Data – Quarterly Financial Data (Unaudited) The following table represents certain unaudited financial information for each of the quarters ended December 31, 2015 and 2014: Three Months Ended (in thousands, except per share data) December 31, 2015 September 30, 2015 June 30, 2015 March 31, 2015 Revenue $ 1,988 $ 1,939 $ 2,043 $ 1,742 Net loss $ (11,487 ) $ (11,870 ) $ (5,806 ) $ (6,211 ) Net loss attributable to common stockholders $ (12,045 ) $ (14,828 ) $ (7,563 ) $ (7,643 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.39 ) $ (14.26 ) $ (7.56 ) $ (7.67 ) Three Months Ended (in thousands, except per share data) December 31, 2014 September 30, 2014 June 30, 2014 March 31, 2014 Revenue $ 1,861 $ 1,915 $ 666 $ 635 Net loss $ (4,787 ) $ (3,589 ) $ (4,930 ) $ (17,004 ) Net loss attributable to common stockholders $ (5,983 ) $ (4,758 ) $ (6,080 ) $ (18,055 ) Net loss per share attributable to common stockholders, basic and diluted $ (6.03 ) $ (4.86 ) $ (6.33 ) $ (18.26 )

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	21. Subsequent Events On January 20, 2016, BMS selected an additional target pursuant to the BMS Agreement entered in May 2014. Under the terms of the BMS Agreement, BMS shall pay the Company a $10 million milestone payment upon the selection of the third target. This amount has been recorded as deferred revenue and will be recognized over the remaining performance period. On February 12, 2016, the Company exercised its option under the ImmunoGen Agreement entered in January 2014 to obtain a worldwide, exclusive, sublicensable license from ImmunoGen for the development and commercialization of products directed against the target selected by the Company under the research collaboration pursuant to the ImmunoGen Agreement. On March 1, 2016, the Company entered into an agreement to terminate the current lease (“Lease Termination”) with its current landlord. The Lease Termination provides for early termination of the current lease effective on November 30, 2016. The Company will not be required to pay the landlord a termination payment in connection with the early termination of the lease. Prior to the execution of the Lease Termination, the current lease had been scheduled to expire on January 31, 2019. The future minimum lease payments for all the Company’s facility leases as of March 1, 2016 are as follows (in thousands): Year Ending December 31: 2016 $ 1,298 2017 3,387 2018 4,374 2019 4,506 2020 and beyond 34,144 Total $ 47,709

Summary of Significant Accoun29

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The Company’s functional and reporting currency is the U.S. dollar.
Use of Estimates	Use of Estimates The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties The Company is subject to a number of risks similar to other biopharmaceutical companies in the early stage, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s products, and protection of proprietary technology. If the Company does not successfully obtain regulatory approval, commercialize or partner any of its product candidates, it will be unable to generate revenue from product sales or achieve profitability. Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents, short term investments and accounts receivable. Substantially all the Company’s cash is held by one financial institution that management believes is of high credit quality. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents in highly rated money market funds and its short-term investments in U.S. Government Bonds. Customers who represent 10% or more of the Company’s total revenue during each period presented or net accounts receivable balance at each respective balance sheet date are as follows: Revenue Accounts Receivable, net For December 31, 2015 2014 2013 2015 2014 Customer A 77 % 54 % * 50 % * Customer B 23 % 46 % 100 % 50 % 92 % * Less than 10%. All of the Company’s customers are located in the United States of America.
Deferred Offering Costs	Deferred Offering Costs Deferred offering costs, which consisted primarily of direct incremental costs related to the Company’s initial public offering of its common stock, were capitalized in other assets until the consummation of the initial public offering. These offering costs were reclassified to additional paid-in capital upon the closing of the initial public offering in October 2015. There were no deferred offering costs capitalized as of December 31, 2014.
Segments	Segments Management has determined that it has one business activity and operates as one operating segment as it only reports financial information on an aggregate basis to its chief executive officer, who is the Company’s chief operating decision maker. All long-lived assets are maintained in the United States of America.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents.
Restricted Cash	Restricted Cash Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015. Restricted cash in 2014 represents amounts related to the security deposit for the Company’s credit card accounts.
Short-term Investments	Short-term Investments All investments have been classified as “available-for-sale” and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities greater than 12 months at the date of purchase are considered long-term investments. Unrealized gains and losses, deemed temporary in nature, are reported as a component of accumulated other comprehensive income (loss), net of tax. A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the security. Premiums (discounts) are amortized (accreted) over the life of the related security as an adjustment to yield using the straight-line interest method. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold.
Property and Equipment, net	Property and Equipment, net Property and equipment are recorded at cost net of accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the respective assets. The useful lives of property and equipment are as follows: Machinery and equipment 5 years Computer equipment and software 3 years Furniture and fixtures 3 years Leasehold improvements Shorter of remaining lease term or estimated life of the assets Maintenance and repairs that do not extend the life or improve the asset are expensed when incurred.
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill represents the excess of the purchase price paid over the fair value of tangible and identifiable intangible assets acquired in business combinations. Goodwill and other intangible assets with indefinite lives are not amortized, but are assigned to reporting units and tested for impairment annually, or whenever there is an impairment indicator. Intangible assets are comprised of in-process research and development (“IPR&D”). The Company assesses impairment indicators annually or more frequently, if a change in circumstances or the occurrence of events suggests the remaining value may not be recoverable. Intangible assets that are not deemed to have an indefinite life are amortized over their estimated useful lives. There was no impairment of goodwill or intangible assets identified during the years ended December 31, 2015 and 2014.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset (or asset group) may not be recoverable and prior to any goodwill impairment test. An impairment loss is recognized when the total of estimated undiscounted future cash flows expected to result from the use of the asset (or asset group) and its eventual disposition is less than its carrying amount. Impairment, if any, would be assessed using discounted cash flows or other appropriate measures of fair value. There was no impairment of long-lived assets during the periods presented in these financial statements.
Convertible Preferred Stock Warrant Liability	Convertible Preferred Stock Warrant Liability Freestanding warrants for shares that are contingently redeemable are classified as liabilities on the balance sheet at their estimated fair value because the shares underlying the warrants may obligate the Company to transfer assets to the holders at a future date under certain circumstances such as a deemed liquidation event. The warrants are subject to re-measurement at each balance sheet date and the change in fair value, if any, is included in other income (expense), net. The Company adjusted the liability for changes in fair value until the consummation of its IPO in October 2015, at which time all convertible preferred stock warrants were net exercised into shares of common stock and the related convertible preferred stock warrant liability was reclassified to additional paid-in capital.
Convertible Preferred Stock Liability	Convertible Preferred Stock Liability The obligation to issue additional shares of Series B-1 and Series C redeemable convertible preferred stock at a future date was determined to be a freestanding instrument that should be accounted for as a liability. At initial recognition, the Company recorded the convertible preferred stock liability on the balance sheets at its estimated fair value. The liability is subject to remeasurement at each balance sheet date, with changes in fair value recognized as a component of other income (expense), net. At the time of each funding, the Company remeasured the liability, with the change in fair value recognized as a component of other income (expense), net and then reclassified the fair value associated with the convertible preferred stock liability to the applicable series of redeemable convertible preferred stock. Immediately prior to the consummation of the Company’s IPO in October 2015, the convertible preferred stock converted to 27,135,453 shares of common stock.
Comprehensive Loss	Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity (deficit) except those resulting from distributions to stockholders. The Company’s unrealized losses on short-term investments represent the only component of other comprehensive loss that is excluded from the reported net loss.
Revenue Recognition	Revenue Recognition The Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; transfer of technology has been completed or services have been rendered; the price to the customer is fixed or determinable and collectability is reasonably assured. The Company’s revenues are primarily derived through its license, research, development and commercialization agreements. The terms of these types of agreements may include (i) licenses to the Company’s technology, (ii) research and development services, and (ii) services or obligations in connection with participation in research or steering committees. Payments to the Company under these arrangements typically include one or more of the following: nonrefundable upfront and license fees, research funding, milestone and other contingent payments to the Company for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products. In arrangements involving the delivery of more than one element, each required deliverable is evaluated to determine whether it qualifies as a separate unit of accounting. The determination is based on whether the deliverable has “standalone value” to the customer. If a deliverable does not qualify as a separate unit of accounting, it is combined with the other applicable undelivered item(s) within the arrangement and these combined deliverables are treated as a single unit of accounting. The arrangement’s consideration that is fixed or determinable is allocated to each separate unit of accounting based on the relative selling price methodology in accordance with the selling price hierarchy, which includes vendor-specific objective evidence (“VSOE”) of selling price, if available, or third-party evidence of selling price if VSOE is not available, or the best estimate of selling price, if neither VSOE nor third-party evidence is available. Payments or reimbursements for the Company’s research and development efforts for the arrangements where such efforts are considered as deliverables are recognized as the services are performed and are presented on a gross basis. When upfront payments are received and if there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, the Company recognizes revenue ratably over the associated period of performance. The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones and sales-based milestones. Such payments are typically payable under the collaborations when the collaboration partner claims or selects a target, or initiates or advances a covered product candidate in preclinical or clinical development, upon submission for marketing approval of a covered product with regulatory authorities, upon receipt of actual marketing approvals of a covered product or for additional indications, or upon the first commercial sale of a covered product. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. Each contingent and milestone payment is evaluated to determine whether it is substantive and at risk to both parties. The Company recognizes any payment that is contingent upon the achievement of a substantive milestone entirely in the period in which the milestone is achieved. Any payments that are contingent upon achievement of a non-substantive milestone are recognized as revenue prospectively, when such payments become due and collectible, over the remaining expected performance period under the arrangement, which is generally the remaining period over which the research and development services are expected to be provided.
Research and Development Expenses	Research and Development Expenses Research and development expenses include costs directly attributable to the conduct of research and development programs, including the cost of salaries, payroll taxes, employee benefits, materials, supplies, depreciation on and maintenance of research equipment, the cost of services provided by outside contractors, and the allocated portions of facility costs, such as rent, utilities, insurance, repairs and maintenance, depreciation, and general support services. All costs associated with research and development are expensed as incurred.
Stock-Based Compensation	Stock-Based Compensation The Company measures its stock-based awards made to employees based on the fair values of the awards as of the grant date using the Black-Scholes option-pricing model. Stock-based compensation expense is recognized over the requisite service period using the straight-line method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. As such, the Company’s stock-based compensation is reduced for the estimated forfeitures at the date of grant and revised in subsequent periods if actual forfeitures differ from those estimates. Stock-based compensation expense for options granted to non-employees as consideration for services received is measured on the date of performance at the fair value of the consideration received or the fair value of the equity instruments issued, using the Black-Scholes option-pricing model, whichever can be more reliably measured. Compensation expense for options granted to non- employees is periodically remeasured as the underlying options vest.
Income Taxes	Income Taxes The Company accounts for income taxes under the liability method which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses and research and development credit carryforwards. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized. The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Net Loss per Share Attributable to Common Stockholders	Net Loss per Share Attributable to Common Stockholders Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration of potentially dilutive securities. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of potentially dilutive securities is anti-dilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU 2014-09, Revenue from Contracts with Customers, which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The ASU will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. The new standard will be effective for the Company on January 1, 2018, which is the effective date for public companies. Early application is permitted as of January 1, 2017. The standard permits the use of either the retrospective or cumulative effect transition method. The Company is evaluating the effect that ASU 2014-09 will have on its financial statements and related disclosures. The Company has not yet selected a transition method nor has it determined the effect of the standard on its ongoing financial reporting. In August 2014, the FASB issued Accounting Standards Update No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern. In 2015, the FASB issued new guidance related to balance sheet classification of deferred taxes. The new guidance requires that deferred tax assets and liabilities be classified as noncurrent in a classified statement of financial position. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. Early adoption is permitted. As the Company’s deferred tax balance is already classified as noncurrent, this new guidance had no financial statement impact during the three months ended December 31, 2015. In February of 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)” (“ASU 2016-02”). Under ASU 2016-2, an entity will be required to recognize right-of-use assets and lease liabilities on its balance sheet and disclose key information about leasing arrangements. ASU 2016-02 offers specific accounting guidance for a lessee, a lessor and sale and leaseback transactions. Lessees and lessors are required to disclose qualitative and quantitative information about leasing arrangements to enable a user of the financial statements to assess the amount, timing and uncertainty of cash flows arising from leases. For public companies, ASU 2016-02 is effective for annual reporting periods beginning after December 15, 2018, including interim periods within that reporting period, and requires a modified retrospective adoption, with early adoption permitted. The Company plans to adopt this guidance beginning with its first quarter ending March 31, 2019. The Company is in the process of evaluating the future impact of ASU 2016-02 on its financial position, results of operations and cash flows.

Summary of Significant Accoun30

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Schedule of Customers Represent 10% or More of Total Revenue or Net Accounts Receivable Balance	Customers who represent 10% or more of the Company’s total revenue during each period presented or net accounts receivable balance at each respective balance sheet date are as follows: Revenue Accounts Receivable, net For December 31, 2015 2014 2013 2015 2014 Customer A 77 % 54 % * 50 % * Customer B 23 % 46 % 100 % 50 % 92 % * Less than 10%.
Schedule of Useful Lives of Property and Equipment	The useful lives of property and equipment are as follows: Machinery and equipment 5 years Computer equipment and software 3 years Furniture and fixtures 3 years Leasehold improvements Shorter of remaining lease term or estimated life of the assets

Fair Value Measurements and S31

Fair Value Measurements and Short-term Investments (Tables)	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Schedule of Financial Assets and Liabilities Subject to Fair Value Measurements on a Recurring Basis	The following tables set forth the fair value of the Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands): December 31, 2015 Level I Level II Level III Total Assets Money market funds $ 44,714 $ — $ — $ 44,714 Restricted cash (money market funds) 917 — — 917 U.S. Government bonds — 140,392 — 140,392 $ 45,631 $ 140,392 $ — $ 186,023 December 31, 2014 Level I Level II Level III Total Assets Restricted cash (money market funds) $ 100 $ — $ — $ 100 $ 100 $ — $ — $ 100 Liabilities Convertible preferred stock warrant liability $ — $ — $ 186 $ 186 Convertible preferred stock liability — — 395 395 $ — $ — $ 581 $ 581
Summary of Changes in the Fair Value of Level 3 Liabilities	The following table sets forth the changes in the fair value of Level III liabilities (in thousands): Convertible Preferred Stock Warrant Liability Convertible Preferred Stock Liability Fair value at December 31, 2014 $ 186 $ 395 Change in fair value 602 1,114 Recognition of fair value upon issuance of redeemable convertible preferred stock (788 ) (1,509 ) Fair value at December 31, 2015 $ — $ —
Summary of Gross Unrealized Gains on the Company's Short-term Investments	The following is a summary of the gross unrealized gains on the Company’s short-term investments (in thousands): December 31, 2015 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Investment Securities U.S. Government bonds $ 126,965 $ — $ (76 ) $ 126,889 Total securities $ 126,965 $ — $ (76 ) $ 126,889
Schedule of Contractual Maturities of Securities Classified as Available-for-sale	The contractual maturities of securities classified as available-for-sale as of December 31, 2015 were as follows (in thousands): December 31, 2015 Due within one year $ 126,889 Total $ 126,889

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31 2015 2014 Machinery and equipment $ 4,910 $ 4,059 Computer equipment and software 452 315 Furniture and fixtures 51 54 Leasehold improvements 720 183 Construction in progress 169 399 6,302 5,010 Less: accumulated depreciation and amortization (2,821 ) (1,992 ) $ 3,481 $ 3,018

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2015
Goodwill And Intangible Assets Disclosure [Abstract]
Schedule of Goodwill and Intangible Assets	Goodwill and intangible assets consisted of the following (in thousands): December 31, 2015 2014 Goodwill $ 949 $ 949 In-process research and development 1,750 1,750

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2015
Accrued Liabilities Current [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): December 31, 2015 2014 Payroll and related expenses $ 2,839 $ 859 Research and clinical expenses 1,562 276 Legal and professional expenses 296 418 Other accrued expenses 215 142 Total $ 4,912 $ 1,695

License Agreement (Tables)

License Agreement (Tables)	12 Months Ended
	Dec. 31, 2015
Research And Development [Abstract]
Schedule of Future Minimum Royalty Obligations Due Under Terms of Certain Exclusive Licensed Patent Rights	The Company has future minimum royalty obligations due under the terms of certain exclusive licensed patent rights. These minimum future obligations are as follows (in thousands): Year ended December 31, 2016 $ 150 2017 150 Total minimum royalty obligations $ 300

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	The future minimum lease payments for all of the Company’s facility leases are as follows (in thousands): Year Ending December 31: 2016 $ 1,369 2017 4,251 2018 5,268 2019 4,582 2020 and beyond 34,144 Total $ 49,614

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Summary of Convertible Preferred Stock	In December 2014, the Company granted a second tranche option (“Second Tranche Option”) to one of its investors to purchase 659,209 shares of its Series C redeemable convertible preferred stock upon the achievement of certain milestones. At initial recognition, the Company recorded the Second Tranche Option as a derivative liability on the balance sheet at its estimated fair value of $395,000. In May 2015, the Company achieved the relevant milestones and the investor exercised their right to purchase 659,209 shares of Series C convertible redeemable preferred stock for net proceeds of $3.5 million. Immediately prior to the closing of this tranche, the Company remeasured the preferred stock liability to its then fair value and recorded a loss from remeasurement of $1.1 million in other income (expense), net. The fair value of the preferred stock liability in the amount of $1.5 million was reclassified to redeemable convertible preferred stock. As of December 31, 2014, the outstanding convertible preferred stock was as follows (in thousands, except share amounts): December 31, 2014 Shares Authorized Shares Issued and Outstanding Net Carrying Value Series A-1 33,101 33,101 $ 49 Series A-2 211,681 211,681 425 Series B-1 14,944,578 14,488,176 57,695 Series B-2 862,412 862,412 2,698 Series C 5,952,664 3,107,701 15,843 Total 22,004,436 18,703,071 $ 76,710

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Schedule Reserved Shares of Common Stock Issuance on Converted Basis	The Company had reserved shares of common stock for issuance, on an as-converted basis, as follows: December 31, 2015 2014 Convertible preferred stock outstanding — 18,703,071 Options issued and outstanding 5,270,751 2,147,872 Convertible preferred stock warrants — 81,620 Shares available for future stock option grants 2,401,406 1,896,617 7,672,157 22,829,180

Stock Option Plans (Tables)

Stock Option Plans (Tables)	12 Months Ended
	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Activity Under Company's Stock Option Plans	Activity under the Company’s stock option plans is set forth below: Options Outstanding Options Available for Grant Number of Options Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Life (years) Aggregate Intrinsic Value (in thousands) Balances at December 31, 2012 1,579,860 940,262 $ 1.197 Options granted (810,390 ) 810,390 1.008 Options exercised — (53,193 ) 1.134 Options forfeited 7,158 (7,158 ) — Retirement of shares under the 2010 Plan — (3,811 ) 1.071 Balances at December 31, 2013 776,628 1,686,490 1.071 8.5 $ 291 Options authorized 1,587,377 — — Options granted (697,557 ) 697,557 1.449 Options exercised — (6,006 ) 1.260 Options forfeited 230,169 (230,169 ) 0.945 Balances at December 31, 2014 1,896,617 2,147,872 1.197 8.1 $ 767 Options authorized 3,801,597 — — Options granted (3,309,708 ) 3,309,708 5.174 Options exercised — (173,929 ) 1.507 Options forfeited 12,900 (12,900 ) 1.405 Balances at December 31, 2015 2,401,406 5,270,751 3.694 8.6 $ 90,542 Options Exercisable—December 31, 2015 1,908,638 1.789 7.2 $ 36,418 Options vested and expected to vest—December 31, 2015 5,206,151 3.685 8.6 $ 89,476
Summary of Stock Options Outstanding and Exercisable by Exercise Price	The following table summarizes information about stock options outstanding and exercisable by exercise price at December 31, 2015: Outstanding Exercisable Exercise Price Number Outstanding Weighted-Average Remaining Contractual Life (Years) Number Exercisable $0.945 411,576 7.16 278,861 $1.134 572,079 5.61 572,079 $1.260 342,793 7.05 306,083 $1.386 164,844 8.05 148,970 $1.449 249,886 8.32 183,100 $1.512 240,058 8.78 81,281 $1.575 815,030 9.13 160,073 $4.473 464,799 9.35 24,629 $6.615 1,966,357 9.63 126,667 $12.00 26,895 9.77 26,895 $21.51 16,434 9.95 — 5,270,751 1,908,638 The following table summarizes information about stock options outstanding and vested by exercise price at December 31, 2014: Outstanding Exercisable Exercise Price Number Outstanding Weighted-Average Remaining Contractual Life (Years) Number Exercisable $0.945 427,329 8.16 214,899 $1.134 661,891 6.59 564,812 $1.260 361,384 8.18 299,106 $1.386 178,072 9.05 81,099 $1.449 269,647 9.34 70,711 $1.512 249,549 9.78 10,582 2,147,872 1,241,209

Stock Based Compensation (Table

Stock Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Total Stock-based Compensation Recorded Related to Option Granted to Employees and Nonemployees	Total stock-based compensation recorded related to option granted to employees and nonemployees was as follows (in thousands): Year Ended December 31, 2015 2014 2013 Research and development $ 1,972 $ 195 $ 121 General and administrative 2,014 358 222 Total stock-based compensation expense $ 3,986 $ 553 $ 343
Schedule of Estimated Fair Value of Employee Stock Options Using Black-Scholes Valuation Model	The Company estimated the fair value of employee stock options using the Black-Scholes valuation model. The fair value of employee stock options is being amortized on a straight-line basis over the requisite service period of the awards. The fair value of employee stock options was estimated using the following assumptions for the year ended December 31, 2015, 2014 and 2013: Year Ended December 31, 2015 2014 2013 Expected volatility 62.9% – 68.9% 66.4% – 71.2% 70.8% – 71.7% Risk-free interest rate 1.4% – 1.9% 1.6% – 2.0% 0.9% – 1.9% Dividend yield — % — % — % Expected term (in years) 5.2 – 7.2 5.3 – 6.1 5.5 – 6.1

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Schedule of Components of Provision (Benefit) for Income Taxes	The Company derives its income only from the United States. The components of the provision (benefit) for income taxes are as follows (in thousands): Years Ended December 31, 2015 2014 2013 Current: Federal $ — $ — $ — State 2 1 1 Total current 2 1 1 Deferred: Federal 8 9 9 State — — — Total deferred 8 9 9 Provision for income taxes $ 10 $ 10 $ 10
Schedule of Effective Tax Rate Reconciliation	A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows: Years Ended December 31, 2015 2014 2013 U.S. federal taxes at statutory rate 34.0 % 34.0 % 34.0 % State tax, net of federal benefit 0.8 % 1.0 % 6.6 % Stock compensation (1.1 )% (0.3 )% (0.5 )% Tax attributes subject to 382 limitation (35.4 )% 0.0 % 0.0 % Tax credits 0.8 % 1.3 % 3.3 % Other (1.8 )% (0.1 )% (0.9 )% Change in valuation allowance 2.7 % (35.9 )% (42.6 )% Total — % — % (0.1 )%
Schedule of Deferred Income Tax Liabilities	The types of temporary differences that give rise to significant portions of the Company’s deferred income tax liabilities are set out below (in thousands): Year Ended December 31, 2015 2014 2013 Net operating loss carryforwards $ 5,688 $ 22,484 $ 13,900 Research and development credits 1,337 2,009 1,324 Intangible—in-process R&D 88 96 124 Deferred revenue 16,182 1,579 — Accruals and deferred rent 998 276 321 Stock-based compensation 1,125 155 86 Other 26 — — Total gross deferred income tax assets 25,444 26,599 15,755 Less: valuation allowance (25,043 ) (26,012 ) (15,130 ) Deferred tax assets, net of valuation allowance 401 587 625 Fixed assets (313 ) (491 ) (419 ) In-process R&D (595 ) (595 ) (697 ) Deferred tax liabilities (908 ) (1,086 ) (1,116 ) Net deferred income tax liabilities $ (507 ) $ (499 ) $ (491 )
Schedule of Reconciliation of Unrecognized Tax Benefits	A reconciliation of the beginning and ending unrecognized tax benefit amount is as follows (in thousands): Year Ended December 31, 2015 2014 2013 Balance at the beginning of the year $ 3,019 $ 532 $ 200 Additions based on tax positions related to current year (2,312 ) 2,473 60 Adjustment based on submitted prior year tax returns (41 ) 14 272 Balance at end of the year $ 666 $ 3,019 $ 532

Net Loss Per Share Attributab42

Net Loss Per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share	The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented, because including them would have been anti-dilutive: Year Ended December 31, 2015 2014 2013 Redeemable convertible preferred stock (on an as-converted basis) 17,507,788 15,024,251 11,995,481 Convertible preferred stock (on an as-converted basis) 192,473 244,782 244,782 Options to purchase common stock 3,865,842 1,987,532 1,482,579 Convertible preferred stock warrants 64,178 81,620 36,559 Total 21,630,281 17,338,185 13,759,401
Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share	A reconciliation of the numerator and denominator used in the calculation of the basic and diluted net loss per share attributable to common stockholders is as follows (in thousands except share and per share amounts): Year Ended December 31, 2015 2014 2013 Numerator: Net loss $ (35,374 ) $ (30,310 ) $ (15,143 ) Add: accretion to redemption value and cumulative dividends on preferred stock (6,705 ) (4,566 ) (3,751 ) Net loss attributable to common stockholders (42,079 ) (34,876 ) (18,894 ) Denominator: Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted 8,595,247 989,453 772,320 Net loss per share attributable to common stockholders, basic and diluted $ (4.90 ) $ (35.25 ) $ (24.46 )

Supplementary Data - Quarterl43

Supplementary Data - Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Unaudited Financial Information	The following table represents certain unaudited financial information for each of the quarters ended December 31, 2015 and 2014: Three Months Ended (in thousands, except per share data) December 31, 2015 September 30, 2015 June 30, 2015 March 31, 2015 Revenue $ 1,988 $ 1,939 $ 2,043 $ 1,742 Net loss $ (11,487 ) $ (11,870 ) $ (5,806 ) $ (6,211 ) Net loss attributable to common stockholders $ (12,045 ) $ (14,828 ) $ (7,563 ) $ (7,643 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.39 ) $ (14.26 ) $ (7.56 ) $ (7.67 ) Three Months Ended (in thousands, except per share data) December 31, 2014 September 30, 2014 June 30, 2014 March 31, 2014 Revenue $ 1,861 $ 1,915 $ 666 $ 635 Net loss $ (4,787 ) $ (3,589 ) $ (4,930 ) $ (17,004 ) Net loss attributable to common stockholders $ (5,983 ) $ (4,758 ) $ (6,080 ) $ (18,055 ) Net loss per share attributable to common stockholders, basic and diluted $ (6.03 ) $ (4.86 ) $ (6.33 ) $ (18.26 )

Subsequent Events (Tables)

Subsequent Events (Tables)	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Schedule of Future Minimum Lease Payments	The future minimum lease payments for all the Company’s facility leases as of March 1, 2016 are as follows (in thousands): Year Ending December 31: 2016 $ 1,298 2017 3,387 2018 4,374 2019 4,506 2020 and beyond 34,144 Total $ 47,709

Description of the Business - A

Description of the Business - Additional Information (Detail) $ / shares in Units, $ in Thousands	Oct. 14, 2015USD ($)$ / sharesshares	Oct. 02, 2015	Oct. 31, 2015USD ($)	Dec. 31, 2015USD ($)shares
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses | $			$ 81,800	$ 81,777
Reverse stock split				one-for-62.997
Reverse stock split conversion ratio		0.01587
Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Issuance of common stock in connection with initial public offering, net of underwriting discount of $6,440 and offering costs of $3,796, share				7,666,667
Initial Public Offering | Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
IPO price per share | $ / shares	$ 12
Issuance of common stock in connection with initial public offering, net of underwriting discount of $6,440 and offering costs of $3,796, share	7,666,667
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses | $	$ 81,800
Underwriters | Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Issuance of common stock in connection with initial public offering, net of underwriting discount of $6,440 and offering costs of $3,796, share	1,000,000

Liquidity - Additional Informat

Liquidity - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	2 Months Ended	12 Months Ended
	Oct. 31, 2015	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accumulated deficit			$ (117,466)	$ (78,138)
Cash, cash equivalents, and short-term investments			186,700
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs			74,430	$ 26,802
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses	$ 81,800		$ 81,777
Series C and Series D Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		$ 73,200

Summary of Significant Accoun47

Summary of Significant Accounting Policies - Schedule of Customers Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details) - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Customer A | Revenue
Concentration Risk [Line Items]
Concentration risk, percentage	77.00%	54.00%
Customer A | Accounts Receivable, net
Concentration Risk [Line Items]
Concentration risk, percentage	50.00%
Customer B | Revenue
Concentration Risk [Line Items]
Concentration risk, percentage	23.00%	46.00%	100.00%
Customer B | Accounts Receivable, net
Concentration Risk [Line Items]
Concentration risk, percentage	50.00%	92.00%

Summary of Significant Accoun48

Summary of Significant Accounting Policies - Schedule of Customers Represent 10% or More of Total Revenue or Net Accounts Receivable Balance (Details) (Parenthetical) - Customer Concentration Risk - Customer B	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue
Concentration Risk [Line Items]
Concentration risk, percentage	23.00%	46.00%	100.00%
Revenue | Maximum
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%
Accounts Receivable, net
Concentration Risk [Line Items]
Concentration risk, percentage	50.00%	92.00%
Accounts Receivable, net | Maximum
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%	10.00%

Summary of Significant Accoun49

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2015USD ($)ActivitySegment	Dec. 31, 2014USD ($)	Oct. 31, 2015shares
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs		$ 0
Number of business activities | Activity	1
Number of operating segments | Segment	1
Impairment of goodwill	$ 0	0
Impairment of intangible assets	0	0
Impairment of long-lived assets	0	$ 0
Convertible preferred stock, shares issued | shares			27,135,453
Interest or penalties charged in relation to unrecognized tax benefits	$ 0
Initial Public Offering | Common Stock
Summary Of Significant Accounting Policies [Line Items]
Convertible preferred stock, shares issued | shares			27,135,453

Summary of Significant Accoun50

Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2015
Machinery and equipment
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	5 years
Computer equipment and software
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Furniture and fixtures
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Leasehold improvements
Property Plant And Equipment [Line Items]
Estimated useful life of leasehold improvements, description	Shorter of remaining lease term or estimated life of the assets

Fair Value Measurements and S51

Fair Value Measurements and Short-term Investments - Schedule of Financial Assets and Liabilities Subject to Fair Value Measurements on a Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Assets, fair value	$ 186,023	$ 100
Liabilities
Liabilities, fair value		581
Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities, fair value		186
Convertible Preferred Stock Liability
Liabilities
Liabilities, fair value		395
Money market funds
Assets
Assets, fair value	44,714
Restricted cash (money market funds)
Assets
Assets, fair value	917	100
U. S. Government bonds
Assets
Assets, fair value	140,392
Level I
Assets
Assets, fair value	45,631	100
Level I | Money market funds
Assets
Assets, fair value	44,714
Level I | Restricted cash (money market funds)
Assets
Assets, fair value	917	100
Level II
Assets
Assets, fair value	140,392
Level II | U. S. Government bonds
Assets
Assets, fair value	$ 140,392
Level III
Liabilities
Liabilities, fair value		581
Level III | Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities, fair value		186
Level III | Convertible Preferred Stock Liability
Liabilities
Liabilities, fair value		$ 395

Fair Value Measurements and S52

Fair Value Measurements and Short-term Investments - Summary of Changes in the Fair Value of Level 3 Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Convertible Preferred Stock Warrant Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value	$ (602)	$ (42)	$ (43)
Convertible Preferred Stock Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value	(1,114)	(13)	$ 110
Level III | Convertible Preferred Stock Warrant Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value, beginning of period	186
Change in fair value	602
Recognition of fair value upon issuance of redeemable convertible preferred stock	(788)
Fair value, end of period		186
Level III | Convertible Preferred Stock Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value, beginning of period	395
Change in fair value	1,114
Recognition of fair value upon issuance of redeemable convertible preferred stock	$ (1,509)
Fair value, end of period		$ 395

Fair Value Measurements and S53

Fair Value Measurements and Short-term Investment - Summary of Gross Unrealized Gains on the Company Short-term Investments (Details) $ in Thousands	Dec. 31, 2015USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 126,965
Gross Unrealized Holding Losses	(76)
Aggregate Fair Value	126,889
U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	126,965
Gross Unrealized Holding Losses	(76)
Aggregate Fair Value	$ 126,889

Fair Value Measurements and S54

Fair Value Measurements and Short-term Investments - Schedule of Contractual Maturities of Securities Classified as Available-for-sale (Details) $ in Thousands	Dec. 31, 2015USD ($)
Fair Value Disclosures [Abstract]
Due within one year	$ 126,889
Total	$ 126,889

Property and Equipment - Summar

Property and Equipment - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 6,302	$ 5,010
Less: accumulated depreciation and amortization	(2,821)	(1,992)
Property and equipment, net	3,481	3,018
Machinery and equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,910	4,059
Computer equipment and software
Property Plant And Equipment [Line Items]
Property and equipment, gross	452	315
Furniture and fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	51	54
Leasehold improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	720	183
Construction In Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 169	$ 399

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 1,206	$ 783	$ 655

Goodwill and Intangible Asset57

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Goodwill And Intangible Assets [Line Items]
Goodwill	$ 949	$ 949
Intangible assets	1,750	1,750
In-process research and development
Goodwill And Intangible Assets [Line Items]
Intangible assets	$ 1,750	$ 1,750

Accrued Liabilities - Summary o

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Accrued Liabilities Current [Abstract]
Payroll and related expenses	$ 2,839	$ 859
Research and clinical expenses	1,562	276
Legal and professional expenses	296	418
Other accrued expenses	215	142
Total	$ 4,912	$ 1,695

Research and Collaboration Ag59

Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details) - Collaborative Arrangement - Pfizer Inc	1 Months Ended		12 Months Ended
	Dec. 31, 2014USD ($)	May. 31, 2013USD ($)TargetResearchTargetSaleIndicator	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets | Target		2
Nominated number of additional research targets | Target		2
Deferred revenue	$ 6,100,000		$ 4,900,000	$ 6,100,000
Estimated performance period of agreement		7 years
Additional target as deferred revenue	1,500,000
Revenue recognized from collaborative arrangement			1,800,000	2,300,000	$ 900,000
Amount due from agreement	$ 1,700,000		$ 400,000	$ 1,700,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 626,500,000
Achievement Of Additional Nominated Research Target One
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		1,500,000
Achievement Of Additional Nominated Research Target Two
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		1,500,000
Exercise of License Options [Member] | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		12,000,000
Achievement Of Development Milestones For Each Research Target Program | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		25,000,000
Achievement Of Development Milestones If Four Research Targets Selected By Counterparty | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 82,000,000
Number of research targets selected | ResearchTarget		4
Achieving First Commercial Sale In Various Territories | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 98,000,000
Number of sales indicators | SaleIndicator		3
Achieving First Commercial Sale If Four Research Targets Selected By Counterparty | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 249,500,000
Number of research targets selected | ResearchTarget		4
Achieving Sales Milestones For Each Research Target Program | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 100,000,000
Achieving Sales Milestones If Four Research Targets Selected By Counterparty | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 280,000,000
Number of research targets selected | ResearchTarget		4
Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received		$ 6,000,000
Deferred revenue		$ 6,000,000

Research and Collaboration Ag60

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details) - Collaborative Arrangement - Immuno Gen Inc	1 Months Ended
	Jan. 31, 2014USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue	$ 13,200,000
Estimated fair value of assets and services	13,200,000
Licenses Received
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services	12,700,000
Research Services, Joint Research Committee Participation and Technology Improvements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services	500,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development and regulatory milestones	30,000,000
Contingent payments receivable upon achieving sales milestones	50,000,000
Contingent payments payable upon achieving development and regulatory milestones	60,000,000
Contingent payments payable upon achieving sales milestones	100,000,000
Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	0
Research License, Research Services, Joint Research Committee, Commercialization License
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue	13,000,000
Technology Improvements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue	$ 200,000

Research and Collaboration Ag61

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)	Jul. 07, 2014USD ($)TargetResearchTargetSaleIndicatorTerm	Oct. 31, 2015shares	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Collaborative Arrangement | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration target research term	2 years
Number of additional collaboration target | Target	2
Research terms			BMS will have additional rights to substitute up to two collaboration targets. Each collaboration target has a two year research term and the two additional targets must be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target can be extended in one year increments up to three times.
Extension of research term for each collaboration target	1 year
Estimated performance period of agreement			10 years
Revenue recognized from collaborative arrangement			$ 5,900,000	$ 2,800,000
Deferred revenue			42,600,000	47,600,000
Amount due from agreement			$ 400,000	$ 100,000
Collaborative Arrangement | Bristol Myers Squibb Company | Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 50,000,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target | Target	4
Number of collaboration target | Target	2
Period of nomination of additional target from effective date	5 years
Times of increments for extended collaboration target research time | Term	3
Contingent milestone payments receivable	$ 1,217,000,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Each Of Two Additional Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	25,000,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achievement Of Development Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	114,000,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achievement Of Development Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 456,000,000
Number of research targets selected | ResearchTarget	4
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving First Commercial Sale In Various Territories
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 124,000,000
Number of sales indicators | SaleIndicator	3
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving First Commercial Sale If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 496,000,000
Number of research targets selected | ResearchTarget	4
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 60,000,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 240,000,000
Number of research targets selected | ResearchTarget	4
Bristol Myers Squibb Agreement | Initial Public Offering
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, shares issued in connection with agreement | shares		833,333

Research and Collaboration Ag62

Research and Collaboration Agreements - MD Anderson - Additional Information (Details)	Nov. 02, 2015
MD Anderson
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option expiration date	Nov. 2, 2019

License Agreement - Additional

License Agreement - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
License Agreement [Line Items]
Common stock, shares issued	36,033,209	996,520
UC Regents
License Agreement [Line Items]
Common stock, shares issued			157,332
Milestone and minimum annual royalty provision paid	$ 347,000	$ 657,000	$ 714,000

License Agreement - Schedule of

License Agreement - Schedule of Future Minimum Royalty Obligations Due Under Terms of Certain Exclusive Licensed Patent Rights (Details) $ in Thousands	Dec. 31, 2015USD ($)
Research And Development [Abstract]
2,016	$ 150
2,017	150
Total minimum royalty obligations	$ 300

Long-term Debt - Additional Inf

Long-term Debt - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Oct. 31, 2015	May. 31, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jan. 31, 2013
Debt Instrument [Line Items]
Warrants issued to purchase stock				81,620
Warrants exercise price			$ 3.084396
Common Stock | Initial Public Offering
Debt Instrument [Line Items]
Convertible preferred stock, shares issued	60,640
Series B-1 Redeemable Convertible Preferred Stock
Debt Instrument [Line Items]
Warrants issued to purchase stock			81,620
Master Loan and Security Agreement
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 2
Line of credit facility, initial basic loan term		42 months
Line of credit facility, additional borrowing capacity					$ 3	$ 0.3
Debt facility interest rate		11.74%

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	Mar. 01, 2016	Dec. 10, 2015USD ($)ft²	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Operating Leased Assets [Line Items]
Lease expiration date			Jan. 31, 2019
Restricted cash			$ 917,000
Rent expense			940,000	$ 836,000	$ 529,000
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit outstanding, amount			900,000
Restricted cash			$ 900,000
Subsequent Event | Lease Termination
Operating Leased Assets [Line Items]
Lease expiration date	Nov. 30, 2016
New Lease Agreement
Operating Leased Assets [Line Items]
Area of rentable office and laboratory space | ft²		76,173
Lease agreement expiration			2,019
Lease term description			The term of the New Lease commences on the later of (i) the date that the Landlord’s construction and tenant improvements have been completed pursuant to the New Lease and (ii) October 1, 2016. The New Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the New Lease.
Initial lease term			10 years
Extended lease term			5 years
Maximum one-time improvement allowance		$ 12,600,000
New Lease Agreement | First year of lease term
Operating Leased Assets [Line Items]
Annual lease rent		3,100,000
New Lease Agreement | Tenth year of lease term
Operating Leased Assets [Line Items]
Annual lease rent		$ 5,500,000
New Lease Agreement | Subsequent Event | Lease Termination
Operating Leased Assets [Line Items]
Lease expiration date	Nov. 30, 2016

Commitments and Contingencies67

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2015USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2,016	$ 1,369
2,017	4,251
2,018	5,268
2,019	4,582
2020 and beyond	34,144
Total	$ 49,614

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional information (Details) - USD ($)	1 Months Ended	12 Months Ended
	May. 31, 2015	Dec. 31, 2015	Oct. 31, 2015	Dec. 31, 2014
Class Of Stock [Line Items]
Shares Issued				18,703,071
Convertible preferred stock liability				$ 395,000
Convertible preferred stock, shares issued			27,135,453
Convertible preferred stock, conversion basis		one-for-one
Shares Outstanding		0		18,703,071
Series C Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Issued	659,209
Convertible preferred stock liability				$ 395,000
Proceeds from issuance of redeemable preferred stock	$ 3,500,000
Fair value of preferred stock liability reclassified to redeemable convertible preferred stock	1,500,000
Series C Redeemable Convertible Preferred Stock | Other Income (Expense), Net
Class Of Stock [Line Items]
Gain (loss) on re-measurement of preferred stock liability	$ 1,100,000
Series C Redeemable Convertible Preferred Stock | Second Tranche Option
Class Of Stock [Line Items]
Preferred stock, shares granted				659,209

Convertible Preferred Stock - S

Convertible Preferred Stock - Summary of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Class Of Stock [Line Items]
Shares Authorized		22,004,436
Shares Issued		18,703,071
Shares Outstanding	0	18,703,071
Net Carrying Value		$ 76,710
Series A-1
Class Of Stock [Line Items]
Shares Authorized		33,101
Shares Issued		33,101
Shares Outstanding		33,101
Net Carrying Value		$ 49
Series A-2
Class Of Stock [Line Items]
Shares Authorized		211,681
Shares Issued		211,681
Shares Outstanding		211,681
Net Carrying Value		$ 425
Series B-1
Class Of Stock [Line Items]
Shares Authorized		14,944,578
Shares Issued		14,488,176
Shares Outstanding		14,488,176
Net Carrying Value		$ 57,695
Series B-2
Class Of Stock [Line Items]
Shares Authorized		862,412
Shares Issued		862,412
Shares Outstanding		862,412
Net Carrying Value		$ 2,698
Series C
Class Of Stock [Line Items]
Shares Authorized		5,952,664
Shares Issued		3,107,701
Shares Outstanding		3,107,701
Net Carrying Value		$ 15,843

Common Stock - Additional Infor

Common Stock - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Oct. 14, 2015
Equity [Abstract]
Common stock, shares authorized	75,000,000	28,572,789	75,000,000
Common stock, par value	$ 0.00001	$ 0.00001	$ 0.00001
Preferred stock, shares authorized	10,000,000	10,000,000	10,000,000
Preferred stock, par value	$ 0.00001	$ 0.00001	$ 0.00001
Dividends on common stock	$ 0	$ 0

Common Stock - Reserved Shares

Common Stock - Reserved Shares of Common Stock Issuance on Converted Basis (Details) - shares	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Shares Outstanding	0	18,703,071
Options issued and outstanding	5,270,751	2,147,872
Warrants issued to purchase stock		81,620
Shares available for future stock option grants	2,401,406	1,896,617
Total	7,672,157	22,829,180

Stock Option Plans - Additional

Stock Option Plans - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period	10 years
Aggregate intrinsic value of stock options exercised	$ 2,300,000	$ 1,500	$ 600
Fair value of options vested	$ 2,800,000	$ 410,000	$ 268,000
Weighted average grant date fair value, options granted	$ 7.169	$ 0.945	$ 0.504
Unrecognized compensation expense, options granted to employees	$ 20,600,000
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Period of weighted average recognition	3 years 6 months
2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period	10 years
Employees holding voting rights of all classes of stock, percentage	10.00%
Estimated fair value of the shares on the date of grant	110.00%
2015 Plan | Share Based Compensation Award Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest period	4 years
Options granted generally vest percent	25.00%
2015 Plan | Vesting per Month After First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent	2.08%
Early Exercise of Employee Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest period	36 months
Early Exercise of Employee Options | Share Based Compensation Award Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent	25.00%

Stock Option Plans - Schedule o

Stock Option Plans - Schedule of Activity Under Company's Stock Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Options Available for Grant
Balances, beginning of the period	1,896,617
Balances, end of the period	2,401,406	1,896,617
Number of Options
Balances, beginning of the period	2,147,872	1,686,490	940,262
Options granted	3,309,708	697,557	810,390
Options exercised	(173,929)	(6,006)	(53,193)
Options forfeited	(12,900)	(230,169)	(7,158)
Retirement of shares under the 2010 Plan			(3,811)
Balances, end of the period	5,270,751	2,147,872	1,686,490
Options Exercisable, end of the period	1,908,638
Options vested and expected to vest, end of the period	5,206,151
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period	$ 1.197	$ 1.071	$ 1.197
Options granted	5.174	1.449	1.008
Options exercised	1.507	1.260	1.134
Options forfeited	1.405	0.945
Retirement of shares under the 2010 Plan			1.071
Balances, end of the period	3.694	$ 1.197	$ 1.071
Options Exercisable, end of the period	1.789
Options vested and expected to vest, end of the period	$ 3.685
Options Outstanding, Weighted- Average Remaining Contractual Life (years)
Balances, end of the period	8 years 7 months 6 days	8 years 1 month 6 days	8 years 6 months
Options Exercisable, end of the period	7 years 2 months 12 days
Options vested and expected to vest, end of the period	8 years 7 months 6 days
Aggregate Intrinsic Value
Balances, end of the period	$ 90,542	$ 767	$ 291
Options Exercisable—December 31, 2015	36,418
Options vested and expected to vest—December 31, 2015	$ 89,476
Employee Stock Option
Options Available for Grant
Balances, beginning of the period	1,896,617	776,628	1,579,860
Options authorized	3,801,597	1,587,377
Options granted	(3,309,708)	(697,557)	(810,390)
Options forfeited	12,900	230,169	7,158
Balances, end of the period	2,401,406	1,896,617	776,628

Stock Option Plans - Summary of

Stock Option Plans - Summary of Stock Options Outstanding and Exercisable by Exercise Price (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 1.789
Options, Number Outstanding	5,270,751	2,147,872	1,686,490	940,262
Options, Weighted-Average Remaining Contractual Life (Years)	8 years 7 months 6 days	8 years 1 month 6 days	8 years 6 months
Options, Number Exercisable	1,908,638
$ 0.945
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 0.945	$ 0.945
Options, Number Outstanding	411,576	427,329
Options, Weighted-Average Remaining Contractual Life (Years)	7 years 1 month 28 days	8 years 1 month 28 days
Options, Number Exercisable	278,861	214,899
$ 1.134
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 1.134	$ 1.134
Options, Number Outstanding	572,079	661,891
Options, Weighted-Average Remaining Contractual Life (Years)	5 years 7 months 10 days	6 years 7 months 2 days
Options, Number Exercisable	572,079	564,812
$ 1.260
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 1.260	$ 1.260
Options, Number Outstanding	342,793	361,384
Options, Weighted-Average Remaining Contractual Life (Years)	7 years 18 days	8 years 2 months 5 days
Options, Number Exercisable	306,083	299,106
$ 1.386
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 1.386	$ 1.386
Options, Number Outstanding	164,844	178,072
Options, Weighted-Average Remaining Contractual Life (Years)	8 years 18 days	9 years 18 days
Options, Number Exercisable	148,970	81,099
$ 1.449
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 1.449	$ 1.449
Options, Number Outstanding	249,886	269,647
Options, Weighted-Average Remaining Contractual Life (Years)	8 years 3 months 26 days	9 years 4 months 2 days
Options, Number Exercisable	183,100	70,711
$ 1.512
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 1.512	$ 1.512
Options, Number Outstanding	240,058	249,549
Options, Weighted-Average Remaining Contractual Life (Years)	8 years 9 months 11 days	9 years 9 months 11 days
Options, Number Exercisable	81,281	10,582
$ 1.575
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 1.575
Options, Number Outstanding	815,030	2,147,872
Options, Weighted-Average Remaining Contractual Life (Years)	9 years 1 month 17 days
Options, Number Exercisable	160,073	1,241,209
$ 4.473
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 4.473
Options, Number Outstanding	464,799
Options, Weighted-Average Remaining Contractual Life (Years)	9 years 4 months 6 days
Options, Number Exercisable	24,629
$ 6.615
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 6.615
Options, Number Outstanding	1,966,357
Options, Weighted-Average Remaining Contractual Life (Years)	9 years 7 months 17 days
Options, Number Exercisable	126,667
$ 12
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 12
Options, Number Outstanding	26,895
Options, Weighted-Average Remaining Contractual Life (Years)	9 years 9 months 7 days
Options, Number Exercisable	26,895
$ 21.51
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price	$ 21.51
Options, Number Outstanding	16,434
Options, Weighted-Average Remaining Contractual Life (Years)	9 years 11 months 12 days

Stock Based Compensation - Tota

Stock Based Compensation - Total Stock-based Compensation Recorded Related to Option Granted to Employees and Nonemployees (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stock-based compensation expense:
Total stock-based compensation expense	$ 3,986	$ 553	$ 343
Research and development
Stock-based compensation expense:
Total stock-based compensation expense	1,972	195	121
General and administrative
Stock-based compensation expense:
Total stock-based compensation expense	$ 2,014	$ 358	$ 222

Stock Based Compensation - Addi

Stock Based Compensation - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock based compensation expense	$ 3,986,000	$ 553,000	$ 343,000
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock based compensation expense	3,200,000	459,000	317,000
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock based compensation expense	$ 773,000	$ 94,000	$ 26,000

Stock Based Compensation-Schedu

Stock Based Compensation-Schedule of Estimated Fair Value of Employee Stock Options Using Black-Scholes Valuation Model (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	62.90%	66.40%	70.80%
Expected volatility, maximum	68.90%	71.20%	71.70%
Risk-free interest rate, minimum	1.40%	1.60%	0.90%
Risk-free interest rate, maximum	1.90%	2.00%	1.90%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 2 months 12 days	5 years 3 months 18 days	5 years 6 months
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	7 years 2 months 12 days	6 years 1 month 6 days	6 years 1 month 6 days

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - Third Rock Ventures - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Related Party Transaction [Line Items]
General and administrative expenses from transactions with related party	$ 33,000	$ 46,000	$ 17,000
Accounts payable, related parties	0	17,600
Principal loan amount, remaining	78,000	404,000
Interest income, related party	$ 4,000	$ 5,000	$ 6,000

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Provision (Benefit) for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current:
State	$ 2	$ 1	$ 1
Total current	2	1	1
Deferred:
Federal	8	9	9
Total deferred	8	9	9
Provision for income taxes	$ 10	$ 10	$ 10

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
U.S. federal taxes at statutory rate	34.00%	34.00%	34.00%
State tax, net of federal benefit	0.80%	1.00%	6.60%
Stock compensation	(1.10%)	(0.30%)	(0.50%)
Tax attributes subject to 382 limitation	(35.40%)	0.00%	0.00%
Tax credits	0.80%	1.30%	3.30%
Other	(1.80%)	(0.10%)	(0.90%)
Change in valuation allowance	2.70%	(35.90%)	(42.60%)
Total			(0.10%)

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Income Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components Of Deferred Tax Assets And Liabilities [Abstract]
Net operating loss carryforwards	$ 5,688	$ 22,484	$ 13,900
Research and development credits	1,337	2,009	1,324
Intangible—in-process R&D	88	96	124
Deferred revenue	16,182	1,579
Accruals and deferred rent	998	276	321
Stock-based compensation	1,125	155	86
Other	26
Total gross deferred income tax assets	25,444	26,599	15,755
Less: valuation allowance	(25,043)	(26,012)	(15,130)
Deferred tax assets, net of valuation allowance	401	587	625
Fixed assets	(313)	(491)	(419)
In-process R&D	(595)	(595)	(697)
Deferred tax liabilities	(908)	(1,086)	(1,116)
Net deferred income tax liabilities	$ (507)	$ (499)	$ (491)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Contingency [Line Items]
Deferred tax asset, net change in total valuation allowance increase (decrease) during period	$ (1,000,000)	$ 10,900,000	$ 6,700,000
Unrecognized tax benefits	666,000	3,019,000	532,000	$ 200,000
Unrecognized tax benefits which would affect company's effective tax rate due to valuation allowance	0
Interest and penalties accrued	$ 0	$ 0	$ 0
Minimum
Income Tax Contingency [Line Items]
Percentage of change in ownership	50.00%
Federal
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 14,300,000
Operating loss carryforwards expiration year	2,031
Federal | 2010
Income Tax Contingency [Line Items]
Open tax year	2,010
Federal | 2014
Income Tax Contingency [Line Items]
Open tax year	2,015
Federal | Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards	$ 400,000
Tax credit carryforward, expiration	2,030
State
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 14,300,000
Operating loss carryforwards expiration year	2,031
State | 2010
Income Tax Contingency [Line Items]
Open tax year	2,010
State | 2014
Income Tax Contingency [Line Items]
Open tax year	2,015
State | Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards	$ 2,100,000

Income Taxes- Schedule of Recon

Income Taxes- Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Balance at the beginning of the year	$ 3,019	$ 532	$ 200
Additions based on tax positions related to current year	(2,312)	2,473	60
Adjustment based on submitted prior year tax returns	(41)	14	272
Balance at end of the year	$ 666	$ 3,019	$ 532

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Compensation And Retirement Disclosure [Abstract]
Company contributions to defined contribution plan	$ 25,000	$ 16,500	$ 9,000

Net Loss Per Share Attributab85

Net Loss Per Share Attributable to Common Stockholders - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	21,630,281	17,338,185	13,759,401
Redeemable convertible preferred stock (on an as-converted basis)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	17,507,788	15,024,251	11,995,481
Convertible preferred stock (on an as-converted basis)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	192,473	244,782	244,782
Convertible preferred stock warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	64,178	81,620	36,559
Options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	3,865,842	1,987,532	1,482,579

Net Loss Per Share Attributab86

Net Loss Per Share Attributable to Common Stockholders - Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share (Details) - 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	$ (11,487)	$ (11,870)	$ (5,806)	$ (6,211)	$ (4,787)	$ (3,589)	$ (4,930)	$ (17,004)	$ (35,374)	$ (30,310)	$ (15,143)
Accretion to redemption value and cumulative dividends on preferred stock									(6,705)	(4,566)	(3,751)
Net loss attributable to common stockholders	$ (12,045)	$ (14,828)	$ (7,563)	$ (7,643)	$ (5,983)	$ (4,758)	$ (6,080)	$ (18,055)	$ (42,079)	$ (34,876)	$ (18,894)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted									8,595,247	989,453	772,320
Net loss per share attributable to common stockholders, basic and diluted									$ (4.90)	$ (35.25)	$ (24.46)

Supplementary Data - Quarterl87

Supplementary Data - Quarterly Financial Data (Unaudited) - Unaudited Financial Information (Details) - 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 Information Disclosure [Abstract]
Revenue	$ 1,988	$ 1,939	$ 2,043	$ 1,742	$ 1,861	$ 1,915	$ 666	$ 635	$ 7,712	$ 5,077	$ 888
Net loss	(11,487)	(11,870)	(5,806)	(6,211)	(4,787)	(3,589)	(4,930)	(17,004)	(35,374)	(30,310)	(15,143)
Net loss attributable to common stockholders	$ (12,045)	$ (14,828)	$ (7,563)	$ (7,643)	$ (5,983)	$ (4,758)	$ (6,080)	$ (18,055)	$ (42,079)	$ (34,876)	$ (18,894)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.39)	$ (14.26)	$ (7.56)	$ (7.67)	$ (6.03)	$ (4.86)	$ (6.33)	$ (18.26)	$ (4.90)	$ (35.25)	$ (24.46)

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - USD ($)	Mar. 01, 2016	Dec. 31, 2015	Jan. 20, 2016
Subsequent Event [Line Items]
Lease expiration date		Jan. 31, 2019
Subsequent Event | Lease Termination
Subsequent Event [Line Items]
Lease expiration date	Nov. 30, 2016
Collaborative Arrangement | Bristol Myers Squibb Company | Subsequent Event | Achieving Third Targets Selected By Counterparty
Subsequent Event [Line Items]
Contingent milestone payments receivable			$ 10,000,000

Subsequent Events - Schedule of

Subsequent Events - Schedule of Future Minimum Lease Payments (Details) - USD ($) $ in Thousands	Mar. 01, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
2,017		$ 4,251
2,018		5,268
2,019		4,582
Total		$ 49,614
Subsequent Event
Subsequent Event [Line Items]
2,016	$ 1,298
2,017	3,387
2,018	4,374
2,019	4,506
2020 and beyond	34,144
Total	$ 47,709