CytomX Therapeutics (CTMX) 10-Q5 May 172017 Q1 Quarterly reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2017	May 03, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Trading Symbol	CTMX
Entity Registrant Name	CytomX Therapeutics, Inc.
Entity Central Index Key	1,501,989
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		36,749,732

CONDENSED BALANCE SHEETS

CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 85,662	$ 104,645
Short-term investments	76,861	77,293
Accounts receivable	213	2,159
Related party accounts receivable	55	154
Prepaid expenses and other current assets	4,118	3,896
Total current assets	166,909	188,147
Property and equipment, net	4,604	4,392
Intangible assets	1,750	1,750
Goodwill	949	949
Restricted cash	917	917
Other assets	2,753	2,973
Total assets	177,882	199,128
Current liabilities:
Accounts payable	3,700	6,596
Accrued liabilities	6,336	8,824
Deferred revenues, current portion	27,090	20,347
Total current liabilities	37,126	35,767
Deferred revenue, net of current portion	65,477	83,803
Deferred tax liability	514	513
Other long-term liabilities	978	566
Total liabilities	104,095	120,649
Commitments and contingencies (Note 10)
Stockholders' equity
Preferred stock, $0.00001 par value; 10,000,000 shares authorized and no shares issued and outstanding at March 31, 2017 and December 31, 2016.
Common stock, $0.00001 par value; 75,000,000 shares authorized; 36,718,940 and 36,490,169 shares issued and outstanding at March 31, 2017 and December 31, 2016, respectively	1	1
Additional paid-in capital	258,509	254,871
Accumulated other comprehensive loss	(100)	(27)
Accumulated deficit	(184,623)	(176,366)
Total stockholders' equity	73,787	78,479
Total liabilities and stockholders' equity	$ 177,882	$ 199,128

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
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	75,000,000
Common stock, shares issued	36,718,940	36,490,169
Common stock, shares outstanding	36,718,940	36,490,169

CONDENSED STATEMENTS OF OPERATI

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 11,176	$ 1,783
Revenues from related parties	477	440
Total revenues	11,653	2,223
Operating expenses:
Research and development	14,576	13,365
General and administrative	5,691	5,040
Total operating expenses	20,267	18,405
Loss from operations	(8,614)	(16,182)
Interest income	442	490
Interest expense	(206)	(353)
Other income (expense), net	120	19
Loss before benefit from (provision for) income taxes	(8,258)	(16,026)
Benefit from (provision for) income taxes	1	(3)
Net loss	$ (8,257)	$ (16,029)
Net loss per share, basic and diluted	$ (0.23)	$ (0.44)
Shares used to compute net loss per share, basic and diluted	36,538,869	36,063,425
Other comprehensive loss:
Changes in unrealized (losses) / gains on short-term investments	$ (73)	$ 106
Total other comprehensive (loss) / income	(73)	106
Comprehensive loss	$ (8,330)	$ (15,923)

CONDENSED STATEMENTS OF CASH FL

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (8,257)	$ (16,029)
Adjustments to reconcile net loss to net cash used in operating activities:
Gain on disposal of property and equipment	(1)
Depreciation and amortization	395	355
Accretion of discount on investments	203	350
Stock-based compensation expense	2,747	2,362
Issuance of stock in connection with services		159
Deferred income taxes	1	3
Changes in operating assets and liabilities
Accounts receivable	1,946	56
Related party accounts receivable	99	214
Prepaid expenses and other current assets	(222)	(1,139)
Other assets	220	55
Accounts payable	(2,825)	246
Accrued liabilities and other long-term liabilities	(1,809)	(371)
Deferred revenue	(11,583)	8,199
Net cash used in operating activities	(19,086)	(5,540)
Cash flows from investing activities:
Purchases of property and equipment	(944)	(327)
Purchases of short-term investments	(40,094)	(33,844)
Maturities of short-term investments	40,250	47,750
Net cash (used in) / provided by investing activities	(788)	13,579
Cash flows from financing activities:
Proceeds from exercise of stock options	891	48
Payment of deferred offering costs		(12)
Net cash provided by financing activities	891	36
Net (decrease) / increase in cash and cash equivalents	(18,983)	8,075
Cash and cash equivalents, beginning of period	104,645	59,822
Cash and cash equivalents, end of period	85,662	67,897
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 230	$ 26

Description of the Business

Description of the Business	3 Months Ended
Description of the Business	Mar. 31, 2017
Disclosure Text Block [Abstract]
Description of the Business	1. Description of the Business CytomX Therapeutics, Inc. (the “Company”) is a clinical-stage, oncology-focused biopharmaceutical company pioneering a novel class of investigational antibody therapeutics based on its Probody technology platform. 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.

Liquidity

Liquidity	3 Months Ended
Liquidity	Mar. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Liquidity	2. Liquidity Since inception, the Company has incurred recurring net operating losses. As of March 31, 2017 and December 31, 2016, the Company had an accumulated deficit of $184.6 million and $176.4 million, respectively, and expects to incur losses for the foreseeable future. To date, the Company has financed its operations primarily through sales of its common stock in conjunction with the Company’s initial public offering (“IPO”), sales of its convertible preferred securities and payments received under its collaboration agreements. As of March 31, 2017 and December 31, 2016, the Company had cash, cash equivalents and short-term investments of $162.5 million and $181.9 million, respectively. The Company expects its existing capital resources will be sufficient to fund our operations for at least twelve months. However , if the anticipated operating results are not achieved in future periods, the Company’s planned expenditures may need to be reduced in order to extend the time period over which the then-available resources would be able to fund the operations. The cost and timing of developing the Company’s products, including CX-072 and CX-2009, are highly uncertain, are subject to substantial risks and many changes. As such, the Company may alter its expenditures as a result of contingencies such as the failure of one of these product candidates in clinical development, the identification of a more promising product candidate in its research efforts or unexpected operating costs and expenditures. The Company will need to raise additional funds in the future. There can be no assurance, however, that such efforts will be successful or that, in the event that they are successful, the terms and conditions of such financing will be favorable to the Company.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
Summary of Significant Accounting Policies	Mar. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	3. Summary of Significant Accounting Policies Basis of Presentation The accompanying interim condensed financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) regarding interim financial reporting. Unaudited Interim Financial Information The accompanying interim condensed financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the results of operations for the periods presented. The year-end condensed balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. The condensed results of operations for the three months ended March 31, 2017 are not necessarily indicative of the results to be expected for the full year or for any other future year or interim period. The accompanying condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016 filed with the SEC. 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 and short-term investments and accounts receivable. Substantially all the Company’s cash is held by one financial institution. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents and short-term investments 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 Three Months Ended March 31, March 31, December 31, 2017 2016 2017 2016 Customer A 56 % — % — % — % Customer B 28 % 80 % 80 % 93 % Customer C 12 % — % — % — % Customer D * 20 % 20 % * * Less than 10% All of the Company’s customers are located in the United States of America. 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. 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 less than 12 months at the date of purchase are considered short-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. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. 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 three months ended March 31, 2017 and the year ended December 31, 2016. 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 interim condensed financial statements. Accrued Research and Development Costs The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and include these costs in accrued liabilities in the balance sheets and within research and development expense in the statements of operations. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Comprehensive Income (Loss) Comprehensive income (loss) represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s unrealized gains and losses on short-term investments represent the only component of other comprehensive income (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 that 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 for the Company’s technology, (ii) research and development services, and (iii) 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 ratable method and is based on the value of the portion of stock-based payment awards. The Company’s stock-based compensation is adjusted in subsequent periods as forfeitures occur. 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. 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 Basic net loss per share is calculated by dividing the net loss 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 is the same as basic net loss per share since the effect of potentially dilutive securities is anti-dilutive. Adopted Accounting Pronouncements Beginning fiscal year 2017, the Company adopted ASU No. 2016-09, Improvements to employee share-based payment, Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) . Under ASU No. 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 No. 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 No. 2016-02 on its financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments . The new standard changes the impairment model for most financial assets and certain other instruments. Under the new standard, entities holding financial assets and net investment in leases that are not accounted for at fair value through net income are to be presented at the net amount expected to be collected. An allowance for credit losses will be a valuation account that will be deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. The new standard will be effective for us on January 1, 2020. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash, Statement of Cash Flows (Topic 230) In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment

Fair Value Measurements and Sho

Fair Value Measurements and Short-Term Investments	3 Months Ended
Fair Value Measurements and Short-Term Investments	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements and Short-Term Investments	4. Fair Value Measurements and Short-Term Investments In accordance with ASC 820-10, Fair Value Measurements and Disclosures, the Company determines the fair value of financial and non-financial assets and liabilities using the fair value hierarchy, which establishes three levels of inputs that may be used to measure 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. The Company’s financial instruments consist of Level I and II assets. Level I assets consist primarily of highly liquid money market funds, some of which 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 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): March 31, 2017 Level I Level II Level III Total Assets Money market funds $ 81,135 $ — $ — $ 81,135 Restricted cash (money market funds) 917 — — 917 U.S. Government bonds — 76,861 — 76,861 Total $ 82,052 $ 76,861 $ — $ 158,913 December 31, 2016 Level I Level II Level III Total Assets Money market funds $ 89,626 $ — $ — $ 89,626 Restricted cash (money market funds) 917 — — 917 U.S. Government bonds — 77,293 — 77,293 Total $ 90,543 $ 77,293 $ — $ 167,836 The following tables set forth the gross unrealized gains and losses on the Company’s investments (in thousands): March 31, 2017 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Investment Securities U.S. Government bonds $ 76,935 $ — $ (74 ) $ 76,861 Total securities $ 76,935 $ — $ (74 ) $ 76,861 December 31, 2016 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Investment Securities U.S. Government bonds $ 77,295 $ 8 $ (10 ) $ 77,293 Total securities $ 77,295 $ 8 $ (10 ) $ 77,293 The following tables set forth the contractual maturities of securities classified as available-for-sale (in thousands): March 31, 2017 Due within one year $ 76,861 Total $ 76,861 December 31, 2016 Due within one year $ 77,293 Total $ 77,293

Property and Equipment

Property and Equipment	3 Months Ended
Property and Equipment	Mar. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment	5. Property and Equipment Property and equipment, net consisted of the following (in thousands): March 31, December 31, 2017 2016 Machinery and equipment $ 6,450 $ 5,973 Computer equipment and software 897 888 Furniture and fixtures 642 651 Leasehold improvements 701 578 Construction in progress 18 45 Total property and equipment 8,708 8,135 Less: accumulated depreciation and amortization (4,104 ) (3,743 ) Property and equipment, net $ 4,604 $ 4,392 Depreciation and amortization expense was $395,000 and $355,000 for three months ended March 31, 2017 and 2016, respectively.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	3 Months Ended
Goodwill and Intangible Assets	Mar. 31, 2017
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	6. Goodwill and Intangible Assets Goodwill and in-process research and development assets resulted 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): March 31, December 31, 2017 2016 Goodwill $ 949 $ 949 In-process research and development 1,750 1,750

Accrued Liabilities

Accrued Liabilities	3 Months Ended
Accrued Liabilities	Mar. 31, 2017
Accrued Liabilities Current [Abstract]
Accrued Liabilities	7. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): March 31, December 31, 2017 2016 Research and clinical expenses $ 3,718 $ 3,909 Payroll and related expenses 1,660 3,971 Legal and professional expenses 705 264 Property and equipment — 331 Other accrued expenses 253 349 Total $ 6,336 $ 8,824

Research and Collaboration Agre

Research and Collaboration Agreements	3 Months Ended
Research and Collaboration Agreements	Mar. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Research and Collaboration Agreements	8. Research and Collaboration Agreements AbbVie Ireland Unlimited Company In April 2016, the Company and AbbVie Ireland Unlimited Company (“AbbVie”) entered into two agreements, a CD71 Co-Development and Licensing Agreement (the “CD71 Agreement”) and a Discovery Collaboration and Licensing Agreement (the “Discovery Agreement” and together with the CD71 Agreement the “AbbVie Agreements”). Under the terms of the CD71 Agreement, the Company and AbbVie will co-develop a Probody Drug Conjugates (“PDC”) against CD71, with the Company responsible for pre-clinical and early clinical development. AbbVie will be responsible for later development and commercialization, with global late-stage development costs shared between the two companies. The Company will assume 35% of the net profits or net losses related to later development unless it opts-out. If the Company opts-out from participation of co-development of the CD71 PDC, AbbVie will have sole right and responsibility for the further development, manufacturing and commercialization of such CD71 PDC. AbbVie, at its sole discretion, may stop development of any CD71 PDC and terminate the CD71 Agreement if the Company does not meet certain preclinical research criteria by the applicable deadline. In such case, the Company and AbbVie may evaluate and approve an alternate CD71 PDC. If such alternate CD71 PDC is approved, then the Company and AbbVie will, in good faith, negotiate amendments to the timelines and, if necessary, the content in the research and development plan and budget and extensions to the deadlines to achieve defined success criteria. Under the CD71 Agreement, the Company received an upfront payment of $20.0 million in April 2016, and is eligible to receive up to $470.0 million in development, regulatory and commercial milestone payments and royalties on ex-US sales in the high teens to low twenties if the Company participates in the co-development of the CD71 Licensed Product subject to a reduction in such royalties if the Company opts-out from the co-development of the CD71 PDC. The Company’s share of later stage co-development costs for each CD71 PDC are capped, provided that AbbVie may offset the Company’s co-development cost above the capped amounts from future payments such as milestone payments and royalties. Under the terms of the Discovery Agreement, AbbVie receives exclusive worldwide rights to develop and commercialize PDC against up to two targets, one of which was selected in March 2017. The Company shall perform research services to discover the Probodies and create PDCs for the nominated collaboration targets. From that point, AbbVie shall have sole right and responsibility for development and commercialization of products comprising or containing such PDCs (“Discovery Licensed Products”). Under the Discovery Agreement, the Company received an upfront payment of $10.0 million in April 2016 and may receive an additional payment upon the selection by AbbVie of the second target and the satisfaction of certain performance conditions under the CD71 Agreement. AbbVie has not selected the second target, but the performance conditions under the CD71 Agreement were met in September 2016. The Company is also eligible to receive up to $275.0 million in target nomination, development, regulatory and commercial milestone payments and royalties in the high single to low teens from commercial sales of any resulting PDCs. The Company has determined that the CD71 and Discovery Agreements with AbbVie should be combined and evaluated as a single arrangement in determining revenue recognition, because both agreements were concurrently negotiated and executed. The Company identified the following deliverables at the inception of the AbbVie Agreements: (1) the research, development and commercialization license for CD71 Probody, (2) the research services related to CD71 Probody, (3) the obligation to participate in the CD71 Agreement joint research committee, (4) the research services related to the first discovery target (5) the research, development and commercialization license for the first discovery target, and (6) the obligation to participate in the Discovery Agreement joint research committee. The Company concluded that, at the inception of the agreement, AbbVie’s option for the second discovery target is substantive and does not represent a deliverable of the agreement. The Company determined that the research, development and commercialization licenses for CD71 and discovery targets do not have a standalone value without the Company’s respective research services and expertise. The Company considered factors such as novelty of the Probody and PDC technology and lack of other parties’ expertise in this space, the Company’s rights to technology relating to a proprietary platform to enable the Probody development and AbbVie’s contractual obligation to use the Company’s research services. The Company also determined that the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee as a single unit of accounting has a standalone value from the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee. Therefore, the Company concluded that there are two units of accounting: CD71 Agreement unit of accounting consisting of the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee, and the Discovery Agreement unit of accounting consisting of the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee. The upfront payments under the AbbVie Agreements are allocated between two units of accounting based on the estimated relative selling prices of each unit. In order to determine the best estimate of selling price, the Company used the discounted cash flow method by calculating risk-adjusted net present values of estimated cash flows. The Company recognizes the allocated amounts ratably over the estimated research service period of five years. The Company recognized revenue of $1.4 million for the three months ended March 31, 2017, related to the AbbVie Agreements. As of March 31, 2017 and December 31, 2016, deferred revenue related to the CD71 unit of accounting was $16.7 million and $17.7 million, respectively, and deferred revenue related to the Discovery Agreement unit of accounting was $8.4 million and $8.9 million, respectively. 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 was initially entitled to receive 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 full-time employee (“FTE”) rate that is capped. The BMS Agreement also required BMS to purchase the Company’s common stock upon an IPO if certain conditions were met. 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, (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 contingent payments under the BMS Agreement relating to development, sales milestone and royalties 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. In January 2016, BMS selected the third target pursuant to the BMS Agreement. Under the terms of the BMS Agreement, BMS paid the Company a $10.0 million payment. In December 2016, BMS selected the fourth and its final target pursuant to the BMS Agreement. Under the terms of the BMS Agreement, BMS paid the Company a $15.0 million payment. Both payments were recorded as deferred revenue and as a result of the fourth target selection, the performance period has been reduced from ten years to seven years and the deferred revenue is being recognized over this new performance period. In December 2016, BMS selected a clinical candidate pursuant to the BMS Agreement, which triggered a $2.0 million pre-clinical milestone payment to the Company. This milestone payment was recognized as revenue in its entirety upon the selection because the achievement of this milestone was based on the Company’s performance. The Company recognized revenue of $3.3 million and $1.8 million for the three months ended March 31, 2017 and 2016, respectively. As of March 31, 2017 and December 31, 2016, deferred revenue relating to the BMS Agreement was $57.8 million and $60.9 million, respectively. The amount due from BMS under the BMS Agreement was $0.2 million and $2.2 million as of March 31, 2017 and December 31, 2016, respectively. On March 17, 2017, the Company and BMS entered into Amendment Number 1 to Extend Collaboration and License Agreement (the “Amendment”). The Amendment grants BMS exclusive worldwide rights to develop and commercialize Probody therapeutics for up to six additional oncology targets and two non-oncology targets. The Company will receive an upfront payment of $200.0 million and will be eligible to receive up to $448.3 million in future development, regulatory and sales milestone payments for each of the targets, as well as tiered royalties from the mid-single digits to low-double digits on net sales of each product commercialized by BMS. 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 a 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, or $6.5 million per target, 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 amounts 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. In February 2017, ImmunoGen exercised its option to obtain a development and commercialization license for one of the two targets under the ImmunoGen Agreement. The Company recognized revenue of $6.5 million and $0 million related to the allocated revenue to the exercise of the development and commercialization license for this target for the three months ended March 31, 2017 and 2016, respectively. As of March 31, 2017 and December 31, 2016, deferred revenue relating to the ImmunoGen Agreement was $6.8 million and $13.2 million, 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 three months ended March 31, 2017 and 2016. 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 PDCs for research project targets nominated by Pfizer. Pfizer nominated two research targets in 2013 and, pursuant to the Pfizer Agreement, had the option of nominating two additional research targets. In December 2014, Pfizer selected an additional research target. The option to select a fourth target lapsed in May 2016. Pfizer has discontinued the epidermal growth factor receptor (“EGFR”) program and has not yet advanced the other two programs to clinical candidate. 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 $6.0 million and is entitled to receive contingent payments of up to an aggregate of $263.5 million as follows: (i) up to $4.5 million upon exercise of the license options, (ii) up to $38.0 million from the achievement of development milestones for the research target programs, (iii) up to $101.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program, and (iv) up to $120.0 million in sales milestones payments for the research target programs. The Company is entitled to receive royalties in the mid-single digit royalties from potential future sales of product candidates. The Company will also receive research and development service fees based on a prescribed 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 the 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 is being recognized over the remaining performance period. Following the lapse of the Pfizer’s option to select a fourth target in May 2016, the amortization period of deferred revenue was adjusted to five and a half years. The Company recognized revenue of $0.5 million and $0.4 million for the three months ended March 31, 2017 and 2016, respectively. As of March 31, 2017 and December 31, 2016, deferred revenue relating to the Pfizer Agreement was $2.9 million and $3.4 million, respectively. The amount due from Pfizer under the Agreement was $0.1 million and $0.1 million as of March 31, 2017 and December 31, 2016, respectively.

License Agreement

License Agreement	3 Months Ended
License Agreement	Mar. 31, 2017
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 to 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 three months ended March 31, 2017 and 2016, the Company incurred expenses of $0.2 million and $0.5 million, respectively, to the UC Regents under the provisions of the UC Agreement. Royalty obligations The Company has annual minimum royalty obligations of $150,000 under the terms of certain exclusive licensed patent rights.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
Commitments and Contingencies	Mar. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Operating Lease Facility Leases On December 10, 2015, the Company entered into a lease (the “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 corporate headquarters. The Company previously leased office and laboratory space located in South San Francisco, California, pursuant to a lease dated March 29, 2013, which expired pursuant to a lease termination agreement (“Lease Termination”) entered into in March 2016. The Lease Termination provided for an early termination of the prior lease and was effective on November 30, 2016. The Company was not required to pay the landlord a termination payment in connection with the early termination of the lease. The term of the Lease commenced on October 1, 2016. The 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 Lease. The Lease provides for annual base rent of approximately $3.1 million in the first year of the lease term. The annual base rent for the second twelve months will be approximately $4.3 million, 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 Lease. The Company has recorded the $0.9 million Letter of Credit in restricted cash as a non-current asset on its balance sheet at March 31, 2017 and December 31, 2016. 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: 2017 (nine months remaining) $ 2,620 2018 4,374 2019 4,506 2020 4,641 2021 and beyond 29,503 Total $ 45,644 Rent expense was $1.1 million for the three months ended March 31, 2017 and a credit of $2,000 for the three months ended March 31, 2016 due to a one-time adjustment to deferred rent pursuant to the termination of the Company’s previous lease for former office and laboratory space. Legal Proceedings The Company is subject to claims and assessments from time to time in the ordinary course of business but is not aware of any such matters, individually or in the aggregate, that 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.

Common Stock

Common Stock	3 Months Ended
Common Stock	Mar. 31, 2017
Equity [Abstract]
Common Stock	11. Common Stock In October 2015, the Company’s board of directors and stockholders approved the Company’s amended and restated certificate of incorporation. The amended and restated certificate of incorporation was effective as of October 14, 2015, and provides for 75,000,000 authorized shares of common stock with a 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 March 31, 2017 and December 31, 2016, no dividends on common stock had been declared by the Board of Directors. The Company had reserved shares of common stock for issuance as follows: March 31, 2017 December 31, 2016 Options issued and outstanding 7,194,792 6,158,746 Shares available for future stock option grants 2,574,918 2,493,188 9,769,710 8,651,934

Stock Option Plans

Stock Option Plans	3 Months Ended
Stock Option Plans	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Option Plans	12. 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”). In conjunction with adopting the 2015 Plan, the Company discontinued the 2011 Plan with respect to new equity awards. The initial number of shares of common stock available for future issuance under the 2015 Plan was 2,444,735. Beginning on January 1, 2016 and continuing until the expiration of the 2015 Plan, the total number of shares of common stock available for issuance under the 2015 Plan will automatically increase annually on January 1 by 4% of the total number of issued and outstanding shares of common stock as of January 1 of the same year. As of March 31, 2017, 2,574,918 shares of common stock were available for future issuance under the 2015 Plan. 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 Number of Options Weighted-Average Exercise Price Per Share Balances at December 31, 2016 6,158,746 $ 5.932 Options granted 1,486,370 11.999 Options exercised (228,771 ) 3.890 Option forfeited/expired (221,553 ) 8.887 Balances at March 31, 2017 7,194,792 $ 7.159 Options exercisable at March 31, 2017 3,173,888 $ 4.185

Employee Stock Purchase Plan

Employee Stock Purchase Plan	3 Months Ended
Employee Stock Purchase Plan	Mar. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Employee Stock Purchase Plan	13. Employee Stock Purchase Plan Concurrent with the completion of the IPO in October 2015, the Company’s Employee Stock Purchase Plan (“ESPP”) became effective. The ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation, subject to any plan limitations. The ESPP generally provides for six-month offering periods, and at the end of each offering period, employees are able to purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last trading day of the offering period. No shares were issued under the ESPP during the three months ended March 31, 2017 and 31,564 shares of common stock were issued in the year ended December 31, 2016. Shares available for future purchase under the ESPP were 683,234 at March 31, 2017. The compensation expense related to the ESPP was $68,000 and $0 for the three months ended March 31, 2017 and 2016, respectively. As of March 31, 2017, there was $45,000 of unrecognized compensation cost related to the ESPP, which we expect to recognize over 2 months.

Stock Based Compensation

Stock Based Compensation	3 Months Ended
Stock Based Compensation	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Based Compensation	14. Stock Based Compensation Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows (in thousands): Three Months Ended March 31, 2017 2016 Stock-based compensation expense: Research and development $ 1,227 $ 1,190 General and administrative 1,520 1,172 Total stock-based compensation expense $ 2,747 $ 2,362

Related Party Transactions

Related Party Transactions	3 Months Ended
Related Party Transactions	Mar. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions Disclosure	15. Related Party Transactions Certain employees of Third Rock Ventures, a stockholder of the Company, provide consulting services to the Company. General and administrative expense for these services of $8,000 and $12,000 were recorded for the three months ended March 31, 2017 and 2016, respectively. The amounts outstanding and included in accounts payable were $8,000 and $12,000 as of March 31, 2017 and December 31, 2016, respectively. Prior to the Company’s IPO, it entered into full recourse loans (“stockholder notes” or “loans”) with current and former executive officers. Principal and interest under these loans was 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’ deficit on the Company’s balance sheets. These loans were repaid and terminated in August 2015 and April 2016. There was no outstanding balance at March 31, 2017 and December 31, 2016. Interest income earned on the loans was immaterial during the three months ended March 31, 2016. Revenues from related parties refer to the collaboration agreement with Pfizer, one of the Company’s stockholders. The Company recognized revenue of $0.5 million and $0.4 million for the three months ended March 31, 2017 and 2016, respectively (Note 8). As of March 31, 2017 and December 31, 2016, deferred revenue relating to the Pfizer Agreement was $2.9 million and $3.4 million, respectively. The amount due from Pfizer under the agreement was $0.1 million and $0.1 million as of March 31, 2017 and December 31, 2016, respectively.

Employee Benefit Plans

Employee Benefit Plans	3 Months Ended
Employee Benefit Plans	Mar. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plans	16. Employee Benefit Plans 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 three months ended March 31, 2017 and 2016, the Company made contributions to the plan of $178,000 and $141,000, respectively.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
Net Loss Per Share	Mar. 31, 2017
Earnings Per Share [Abstract]
Net Loss Per Share	17. 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 for the periods presented, because including them would have been anti-dilutive: Three Months Ended March 31, 2017 2016 Options to purchase common stock 7,055,207 5,951,381 Total 7,055,207 5,951,381 A reconciliation of the numerator and denominator used in the calculation of the basic and diluted net loss per share is as follows (in thousands except share and per share amounts): Three Months Ended March 31, 2017 2016 Numerator: Net loss (8,257 ) (16,029 ) Denominator: Weighted-average common shares outstanding used to calculate net loss per share, basic and diluted 36,538,869 36,063,425 Net loss per share, basic and diluted $ (0.23 ) $ (0.44 )

Subsequent Event

Subsequent Event	3 Months Ended
Subsequent Event	Mar. 31, 2017
Subsequent Events [Abstract]
Subsequent Event	18. Subsequent Event The Amendment the Company entered into with BMS on March 17, 2017 (Note 8) became effective on April 24, 2017 under the Hart-Scott-Rodino Antitrust Improvements Act. The Company will receive $200.0 million from Bristol-Myers Squibb pursuant to the Amendment.

Summary of Significant Accoun24

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying interim condensed financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) regarding interim financial reporting.
Unaudited Interim Financial Information	Unaudited Interim Financial Information The accompanying interim condensed financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the results of operations for the periods presented. The year-end condensed balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. The condensed results of operations for the three months ended March 31, 2017 are not necessarily indicative of the results to be expected for the full year or for any other future year or interim period. The accompanying condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016 filed with the SEC.
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 and short-term investments and accounts receivable. Substantially all the Company’s cash is held by one financial institution. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents and short-term investments 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 Three Months Ended March 31, March 31, December 31, 2017 2016 2017 2016 Customer A 56 % — % — % — % Customer B 28 % 80 % 80 % 93 % Customer C 12 % — % — % — % Customer D * 20 % 20 % * * Less than 10% All of the Company’s customers are located in the United States of America.
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.
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 less than 12 months at the date of purchase are considered short-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. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. 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 three months ended March 31, 2017 and the year ended December 31, 2016.
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 interim condensed financial statements.
Accrued Research and Development Costs	Accrued Research and Development Costs The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and include these costs in accrued liabilities in the balance sheets and within research and development expense in the statements of operations. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. 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.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s unrealized gains and losses on short-term investments represent the only component of other comprehensive income (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 that 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 for the Company’s technology, (ii) research and development services, and (iii) 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.
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 ratable method and is based on the value of the portion of stock-based payment awards. The Company’s stock-based compensation is adjusted in subsequent periods as forfeitures occur. 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. 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	Net Loss per Share Basic net loss per share is calculated by dividing the net loss 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 is the same as basic net loss per share since the effect of potentially dilutive securities is anti-dilutive.
Adopted and Recent Accounting Pronouncements	Adopted Accounting Pronouncements Beginning fiscal year 2017, the Company adopted ASU No. 2016-09, Improvements to employee share-based payment, Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) . Under ASU No. 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 No. 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 No. 2016-02 on its financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments . The new standard changes the impairment model for most financial assets and certain other instruments. Under the new standard, entities holding financial assets and net investment in leases that are not accounted for at fair value through net income are to be presented at the net amount expected to be collected. An allowance for credit losses will be a valuation account that will be deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. The new standard will be effective for us on January 1, 2020. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash, Statement of Cash Flows (Topic 230) In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Tables)	3 Months Ended
Summary of Significant Accounting Policies (Tables)	Mar. 31, 2017
Accounting Policies [Abstract]
Schedule of Customers who 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 Three Months Ended March 31, March 31, December 31, 2017 2016 2017 2016 Customer A 56 % — % — % — % Customer B 28 % 80 % 80 % 93 % Customer C 12 % — % — % — % Customer D * 20 % 20 % * * 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 S26

Fair Value Measurements and Short-Term Investments (Tables)	3 Months Ended
Fair Value Measurements and Short-Term Investments (Tables)	Mar. 31, 2017
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): March 31, 2017 Level I Level II Level III Total Assets Money market funds $ 81,135 $ — $ — $ 81,135 Restricted cash (money market funds) 917 — — 917 U.S. Government bonds — 76,861 — 76,861 Total $ 82,052 $ 76,861 $ — $ 158,913 December 31, 2016 Level I Level II Level III Total Assets Money market funds $ 89,626 $ — $ — $ 89,626 Restricted cash (money market funds) 917 — — 917 U.S. Government bonds — 77,293 — 77,293 Total $ 90,543 $ 77,293 $ — $ 167,836
Schedule of Gross Unrealized Gains and Losses on the Company's Investments	The following tables set forth the gross unrealized gains and losses on the Company’s investments (in thousands): March 31, 2017 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Investment Securities U.S. Government bonds $ 76,935 $ — $ (74 ) $ 76,861 Total securities $ 76,935 $ — $ (74 ) $ 76,861 December 31, 2016 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Investment Securities U.S. Government bonds $ 77,295 $ 8 $ (10 ) $ 77,293 Total securities $ 77,295 $ 8 $ (10 ) $ 77,293
Schedule of Contractual Maturities of Securities Classified as Available-for-sale	The following tables set forth the contractual maturities of securities classified as available-for-sale (in thousands): March 31, 2017 Due within one year $ 76,861 Total $ 76,861 December 31, 2016 Due within one year $ 77,293 Total $ 77,293

Property and Equipment (Tables)

Property and Equipment (Tables)	3 Months Ended
Property and Equipment (Tables)	Mar. 31, 2017
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): March 31, December 31, 2017 2016 Machinery and equipment $ 6,450 $ 5,973 Computer equipment and software 897 888 Furniture and fixtures 642 651 Leasehold improvements 701 578 Construction in progress 18 45 Total property and equipment 8,708 8,135 Less: accumulated depreciation and amortization (4,104 ) (3,743 ) Property and equipment, net $ 4,604 $ 4,392

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	3 Months Ended
Goodwill and Intangible Assets (Tables)	Mar. 31, 2017
Goodwill And Intangible Assets Disclosure [Abstract]
Schedule of Goodwill and Intangible Assets	Goodwill and intangible assets consisted of the following (in thousands): March 31, December 31, 2017 2016 Goodwill $ 949 $ 949 In-process research and development 1,750 1,750

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	3 Months Ended
Accrued Liabilities (Tables)	Mar. 31, 2017
Accrued Liabilities Current [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): March 31, December 31, 2017 2016 Research and clinical expenses $ 3,718 $ 3,909 Payroll and related expenses 1,660 3,971 Legal and professional expenses 705 264 Property and equipment — 331 Other accrued expenses 253 349 Total $ 6,336 $ 8,824

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
Commitments and Contingencies (Tables)	Mar. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Schedule of 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: 2017 (nine months remaining) $ 2,620 2018 4,374 2019 4,506 2020 4,641 2021 and beyond 29,503 Total $ 45,644

Common Stock (Tables)

Common Stock (Tables)	3 Months Ended
Common Stock (Tables)	Mar. 31, 2017
Equity [Abstract]
Schedule Reserved Shares of Common Stock Issuance on Converted Basis	The Company had reserved shares of common stock for issuance as follows: March 31, 2017 December 31, 2016 Options issued and outstanding 7,194,792 6,158,746 Shares available for future stock option grants 2,574,918 2,493,188 9,769,710 8,651,934

Stock Option Plans (Tables)

Stock Option Plans (Tables)	3 Months Ended
Stock Option Plans (Tables)	Mar. 31, 2017
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 Number of Options Weighted-Average Exercise Price Per Share Balances at December 31, 2016 6,158,746 $ 5.932 Options granted 1,486,370 11.999 Options exercised (228,771 ) 3.890 Option forfeited/expired (221,553 ) 8.887 Balances at March 31, 2017 7,194,792 $ 7.159 Options exercisable at March 31, 2017 3,173,888 $ 4.185

Stock Based Compensation (Table

Stock Based Compensation (Tables)	3 Months Ended
Stock Based Compensation (Tables)	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Total Stock-based Compensation Recognized	Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows (in thousands): Three Months Ended March 31, 2017 2016 Stock-based compensation expense: Research and development $ 1,227 $ 1,190 General and administrative 1,520 1,172 Total stock-based compensation expense $ 2,747 $ 2,362

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
Net Loss Per Share (Tables)	Mar. 31, 2017
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 for the periods presented, because including them would have been anti-dilutive: Three Months Ended March 31, 2017 2016 Options to purchase common stock 7,055,207 5,951,381 Total 7,055,207 5,951,381
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 is as follows (in thousands except share and per share amounts): Three Months Ended March 31, 2017 2016 Numerator: Net loss (8,257 ) (16,029 ) Denominator: Weighted-average common shares outstanding used to calculate net loss per share, basic and diluted 36,538,869 36,063,425 Net loss per share, basic and diluted $ (0.23 ) $ (0.44 )

Liquidity - Additional Informat

Liquidity - Additional Information (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ (184,623)	$ (176,366)
Cash, cash equivalents, and short-term investments	$ 162,500	$ 181,900

Summary of Significant Accoun36

Summary of Significant Accounting Policies - Schedule of Customers who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details) - Customer Concentration Risk	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Revenue \| Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	56.00%
Revenue \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	28.00%	80.00%
Revenue \| Customer C
Concentration Risk [Line Items]
Concentration risk, percentage	12.00%
Revenue \| Customer D
Concentration Risk [Line Items]
Concentration risk, percentage		20.00%
Accounts Receivable, net \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	80.00%		93.00%
Accounts Receivable, net \| Customer D
Concentration Risk [Line Items]
Concentration risk, percentage	20.00%

Summary of Significant Accoun37

Summary of Significant Accounting Policies - Schedule of Customers who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details) (Parenthetical) - Customer Concentration Risk - Customer D	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Revenue
Concentration Risk [Line Items]
Concentration risk, percentage		20.00%
Revenue \| Maximum
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%
Accounts Receivable, net
Concentration Risk [Line Items]
Concentration risk, percentage	20.00%
Accounts Receivable, net \| Maximum
Concentration Risk [Line Items]
Concentration risk, percentage			10.00%

Summary of Significant Accoun38

Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2017USD ($)ActivitySegment	Mar. 31, 2016USD ($)	Dec. 31, 2016USD ($)
Accounting Policies [Abstract]
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
Interest or penalties charged in relation to unrecognized tax benefits	$ 0

Summary of Significant Accoun39

Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	3 Months Ended
	Mar. 31, 2017
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 S40

Fair Value Measurements and Short-Term Investments - Schedule of Financial Assets and Liabilities Subject to Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets
Assets, fair value	$ 158,913	$ 167,836
Money market funds
Assets
Assets, fair value	81,135	89,626
Restricted cash (money market funds)
Assets
Assets, fair value	917	917
U. S. Government bonds
Assets
Assets, fair value	76,861	77,293
Level I
Assets
Assets, fair value	82,052	90,543
Level I \| Money market funds
Assets
Assets, fair value	81,135	89,626
Level I \| Restricted cash (money market funds)
Assets
Assets, fair value	917	917
Level II
Assets
Assets, fair value	76,861	77,293
Level II \| U. S. Government bonds
Assets
Assets, fair value	$ 76,861	$ 77,293

Fair Value Measurements and S41

Fair Value Measurements and Short-Term Investments - Schedule of Gross Unrealized Gains and Losses on the Company's Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 76,935	$ 77,295
Gross Unrealized Holding Gains		8
Gross Unrealized Holding Losses	(74)	(10)
Aggregate Fair Value	76,861	77,293
U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	76,935	77,295
Gross Unrealized Holding Gains		8
Gross Unrealized Holding Losses	(74)	(10)
Aggregate Fair Value	$ 76,861	$ 77,293

Fair Value Measurements and S42

Fair Value Measurements and Short-Term Investments - Schedule of Contractual Maturities of Securities Classified as Available-for-sale (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Due within one year	$ 76,861	$ 77,293
Total	$ 76,861	$ 77,293

Property and Equipment - Summar

Property and Equipment - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment	$ 8,708	$ 8,135
Less: accumulated depreciation and amortization	(4,104)	(3,743)
Property and equipment, net	4,604	4,392
Machinery and equipment
Property Plant And Equipment [Line Items]
Total property and equipment	6,450	5,973
Computer equipment and software
Property Plant And Equipment [Line Items]
Total property and equipment	897	888
Furniture and fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	642	651
Leasehold improvements
Property Plant And Equipment [Line Items]
Total property and equipment	701	578
Construction in progress
Property Plant And Equipment [Line Items]
Total property and equipment	$ 18	$ 45

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 395	$ 355

Goodwill and Intangible Asset45

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill	$ 949	$ 949
In-process research and development	$ 1,750	$ 1,750

Accrued Liabilities - Summary o

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Accrued Liabilities Current [Abstract]
Research and clinical expenses	$ 3,718	$ 3,909
Payroll and related expenses	1,660	3,971
Legal and professional expenses	705	264
Property and equipment		331
Other accrued expenses	253	349
Total	$ 6,336	$ 8,824

Research and Collaboration Ag47

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details) - AbbVie Ireland Unlimited Company	1 Months Ended	3 Months Ended
	Apr. 30, 2016USD ($)AgreementTargetAccountingUnit	Mar. 31, 2017USD ($)Target	Dec. 31, 2016USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of accounting units \| AccountingUnit	2
Estimated research service period		5 years
Revenue recognized from collaborative arrangement		$ 1,400,000
Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of collaboration agreements \| Agreement	2
Percentage of net profits or net losses related to development costs	35.00%
Number of targets selected \| Target		1
Collaborative Arrangement \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 10,000,000
Collaborative Arrangement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets \| Target	2
Collaborative Arrangement \| Maximum \| Development, Regulatory and Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 275,000,000
CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue		$ 16,700,000	$ 17,700,000
CD71 Agreement \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	20,000,000
CD71 Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development, regulatory and commercial milestones	$ 470,000,000
Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue		$ 8,400,000	$ 8,900,000

Research and Collaboration Ag48

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)	Mar. 17, 2017USD ($)Target	Jul. 07, 2014USD ($)TargetTermResearchTargetSaleIndicator	Oct. 31, 2015shares	Mar. 31, 2017USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2016USD ($)	Jan. 31, 2016USD ($)
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
Contingent milestone payments receivable						$ 15,000,000	$ 10,000,000
Estimated performance period of agreement				10 years
Revised estimated performance period of agreement				7 years
Revenue recognized from collaborative arrangement				$ 3,300,000	$ 1,800,000
Deferred revenue				57,800,000		60,900,000
Amount due from agreement				$ 200,000		2,200,000
Collaborative Arrangement \| Bristol Myers Squibb Company \| Clinical Candidate
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable						$ 2,000,000
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
Collaboration and License Agreement \| Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of non-oncology target \| Target	2
Collaboration and License Agreement \| Bristol Myers Squibb Company \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 200,000,000
Collaboration and License Agreement \| Maximum \| Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target \| Target	6
Contingent payments receivable upon achieving development, regulatory and commercial milestones	$ 448,300,000

Research and Collaboration Ag49

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details) - Collaborative Arrangement - Immuno Gen Inc - USD ($)	1 Months Ended	3 Months Ended
	Jan. 31, 2014	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue	$ 13,200,000	$ 6,800,000		$ 13,200,000
Estimated fair value of assets and services	13,200,000
Revenue recognized from collaborative arrangement		$ 6,500,000	$ 0
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
Estimated total fair value consideration recorded as deferred revenue, per target	6,500,000
Technology Improvements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue	$ 200,000

Research and Collaboration Ag50

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

Research and Collaboration Ag51

Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details) - Collaborative Arrangement - Pfizer Inc	1 Months Ended			3 Months Ended
	May 31, 2016	Dec. 31, 2014USD ($)	May 31, 2013USD ($)TargetSaleIndicator	Mar. 31, 2017USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2016USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets \| Target			2
Nominated number of additional research targets \| Target			2
Deferred revenue				$ 2,900,000		$ 3,400,000
Estimated performance period of agreement			7 years
Additional target as deferred revenue		$ 1,500,000
Adjusted amortization period of deferred revenue	5 years 6 months
Revenue recognized from collaborative arrangement				500,000	$ 400,000
Amount due from agreement				$ 100,000		$ 100,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			$ 263,500,000
Exercise of License Options \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			4,500,000
Achievement Of Development Milestones For Each Research Target Program \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			38,000,000
Achieving First Commercial Sale In Various Territories \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			$ 101,000,000
Number of sales indicators \| SaleIndicator			3
Achieving Sales Milestones For Each Research Target Program \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			$ 120,000,000
Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			6,000,000
Deferred revenue			$ 6,000,000

License Agreement - Additional

License Agreement - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2013
License Agreement [Line Items]
Common stock, shares issued	36,718,940		36,490,169
Annual minimum royalty obligations	$ 150,000
UC Regents
License Agreement [Line Items]
Common stock, shares issued				157,332
Milestone and minimum annual royalty provision paid	$ 200,000	$ 500,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	Dec. 10, 2015USD ($)ft²	Mar. 31, 2016USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Operating Leased Assets [Line Items]
Restricted cash			$ 917,000	$ 917,000
Rent expense			1,100,000
One-time adjustment to deferred rent		$ 2,000
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit outstanding, amount			900,000
Restricted cash			$ 900,000	$ 900,000
New Lease Agreement
Operating Leased Assets [Line Items]
Area of rentable office and laboratory space \| ft²	76,173
Lease term description			The term of the Lease commenced on October 1, 2016. The 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 Lease.
Initial lease term			10 years
Extended lease term			5 years
Maximum one-time improvement allowance	$ 12,600,000
Improvement allowance from recoverable rent	2,300,000
New Lease Agreement \| First year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	3,100,000
New Lease Agreement \| Second twelve months of lease term
Operating Leased Assets [Line Items]
Annual lease rent	4,300,000
New Lease Agreement \| Tenth year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	$ 5,500,000
Amendment to Current Lease \| Early Lease Termination
Operating Leased Assets [Line Items]
Lease expiration date		Nov. 30, 2016

Commitments and Contingencies54

Commitments and Contingencies - Schedule of Minimum Lease Payments (Details) $ in Thousands	Mar. 31, 2017USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2017 (nine months remaining)	$ 2,620
2,018	4,374
2,019	4,506
2,020	4,641
2021 and beyond	29,503
Total	$ 45,644

Common Stock - Additional Infor

Common Stock - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
Common Stock - Additional Information (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016	Oct. 14, 2015
Equity [Abstract]
Common stock, shares authorized	75,000,000	75,000,000	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	Mar. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Options issued and outstanding	7,194,792	6,158,746
Shares available for future stock option grants	2,574,918	2,493,188
Total	9,769,710	8,651,934

Stock Option Plans - Additional

Stock Option Plans - Additional Information (Details) - shares	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period	10 years
Common stock, capital shares reserved for future issuance	9,769,710	8,651,934
Vesting per Month After First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent	2.08%
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%
Common stock, capital shares reserved for future issuance	2,574,918		2,444,735
Aggregate intrinsic value of stock options exercised		4.00%
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%
Minimum \| 2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of the shares on the date of grant	110.00%

Stock Option Plans - Schedule o

Stock Option Plans - Schedule of Activity Under Company's Stock Option Plans (Details)	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Number of Options
Balances, beginning of the period \| shares	6,158,746
Options granted \| shares	1,486,370
Options exercised \| shares	(228,771)
Option forfeited/expired \| shares	(221,553)
Balances, end of the period \| shares	7,194,792
Options exercisable, end of the period \| shares	3,173,888
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period \| $ / shares	$ 5.932
Options granted \| $ / shares	11.999
Options exercised \| $ / shares	3.890
Option forfeited/expired \| $ / shares	8.887
Balances, end of the period \| $ / shares	7.159
Options exercisable, end of the period \| $ / shares	$ 4.185

Employee Stock Purchase Plan -

Employee Stock Purchase Plan - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Shares available for future purchase employee stock purchase plan	9,769,710		8,651,934
Stock-based compensation expense	$ 2,747,000	$ 2,362,000
Employee Stock Purchase Plan
Defined Contribution Plan Disclosure [Line Items]
Plan maximum percentage of eligible compensation to purchase shares	15.00%
Plan percentage of fair market value, employee share purchases	85.00%
Number of shares issued during period	0		31,564
Shares available for future purchase employee stock purchase plan	683,234
Stock-based compensation expense	$ 68,000	$ 0
Unrecognized stock-based compensation cost	$ 45,000

Stock Based Compensation - Tota

Stock Based Compensation - Total Stock-based Compensation Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Stock-based compensation expense:
Stock-based compensation expense	$ 2,747	$ 2,362
Research and development
Stock-based compensation expense:
Stock-based compensation expense	1,227	1,190
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 1,520	$ 1,172

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Collaborative Arrangement \| Pfizer Inc
Related Party Transaction [Line Items]
Revenue recognized	$ 500,000	$ 400,000
Deferred revenue	2,900,000		$ 3,400,000
Amount due from agreement	100,000		100,000
Third Rock Ventures
Related Party Transaction [Line Items]
General and administrative expense from transactions with related party	8,000	$ 12,000
Accounts payable, related parties	8,000		12,000
Principal loan amount, outstanding	$ 0		$ 0

Employee Benefit Plans- Additio

Employee Benefit Plans- Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Company contributions to defined contribution plan	$ 178,000	$ 141,000

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	7,055,207	5,951,381
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	7,055,207	5,951,381

Net Loss Per Share - Reconcilia

Net Loss Per Share - 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
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net loss	$ (8,257)	$ (16,029)
Denominator:
Shares used to compute net loss per share, basic and diluted	36,538,869	36,063,425
Net loss per share, basic and diluted	$ (0.23)	$ (0.44)

Subsequent Event - Additional I

Subsequent Event - Additional Information (Details) $ in Millions	Apr. 24, 2017USD ($)
Subsequent Event \| Bristol Myers Squibb Agreement
Subsequent Event [Line Items]
Contingent milestone payments receivable	$ 200