CytomX Therapeutics (CTMX) 10-Q 9 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	Apr. 30, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	CTMX
Entity Registrant Name	CytomX Therapeutics, Inc.
Entity Central Index Key	0001501989
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		45,311,464

CONDENSED BALANCE SHEETS

CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	[1]
Current assets:
Cash and cash equivalents	$ 190,407	$ 247,577
Short-term investments	206,189	188,550
Accounts receivable	44	97
Prepaid expenses and other current assets	8,437	9,251
Total current assets	405,077	445,475
Property and equipment, net	7,369	6,934
Intangible assets, net	1,422	1,458
Goodwill	949	949
Restricted cash	917	917
Operating lease right-of-use	27,404
Other assets	1,375	1,375
Total assets	444,513	457,108
Current liabilities:
Accounts payable	9,511	5,132
Accrued liabilities	22,567	26,724
Income tax payable	13,374	13,339
Deferred revenue, current portion	50,765	52,713
Total current liabilities	96,217	97,908
Deferred revenue, net of current portion	197,754	225,267
Operating lease liabilities - long term	27,008
Other long-term liabilities	963	3,050
Total liabilities	321,942	326,225
Commitments and contingencies
Stockholders' equity:
Common stock, $0.00001 par value; 75,000,000 shares authorized; 45,157,652 and 45,083,209 shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively	1	1
Additional paid-in capital	451,613	445,956
Accumulated other comprehensive income (loss)	73	(93)
Accumulated deficit	(329,116)	(314,981)
Total stockholders' equity	122,571	130,883
Total liabilities and stockholders' equity	444,513	457,108
Convertible Preferred Stock
Stockholders' equity:
Convertible preferred stock, $0.00001 par value; 10,000,000 shares authorized and no shares issued and outstanding at March 31, 2019 and December 31, 2018.
[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	45,157,652	45,083,209
Common stock, shares outstanding	45,157,652	45,083,209
Convertible Preferred Stock
Convertible Preferred stock, par value	$ 0.00001	$ 0.00001
Convertible Preferred stock, shares authorized	10,000,000	10,000,000
Convertible Preferred stock, shares issued	0	0
Convertible Preferred stock, shares outstanding	0	0

CONDENSED STATEMENTS OF OPERATI

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 29,485	$ 14,184
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 36,376	$ 22,458
General and administrative	9,674	7,356
Total operating expenses	46,050	29,814
Loss from operations	(16,565)	(15,630)
Interest income	2,496	1,375
Other expense	(61)	(140)
Loss before income taxes	(14,130)	(14,395)
Provision for (benefit from) income taxes	(6)	1,098
Net loss	$ (14,124)	$ (15,493)
Net loss per share, basic and diluted	$ (0.31)	$ (0.40)
Shares used to compute net loss per share, basic and diluted	45,122,456	38,647,878
Other comprehensive income (loss):
Changes in unrealized gain (loss) on short-term investments, net of tax	$ 155	$ (134)
Impact of adoption of new accounting pronouncement	11
Comprehensive loss	$ (13,958)	$ (15,627)

STATEMENTS OF STOCKHOLDERS' EQU

STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total		Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ 69,896		$ 1	$ 289,454	$ (94)	$ (219,465)
Beginning balance, shares at Dec. 31, 2017			38,478,560
Impact of adoption of new accounting pronouncement | ASC 606	(10,912)					(10,912)
Exercise of stock options	2,602			2,602
Exercise of stock options, shares			425,139
Stock-based compensation	3,688			3,688
Other comprehensive income	(134)				(134)
Net loss	(15,493)					(15,493)
Ending balance at Mar. 31, 2018	49,647		$ 1	295,744	(228)	(245,870)
Ending balance, shares at Mar. 31, 2018			38,903,699
Beginning balance at Dec. 31, 2018	130,883	[1]	$ 1	445,956	(93)	(314,981)
Beginning balance, shares at Dec. 31, 2018			45,083,209
Impact of adoption of new accounting pronouncement | ASU 2018-02	11,000				11	(11)
Exercise of stock options	$ 465			465
Exercise of stock options, shares	74,443		74,443
Stock-based compensation	$ 5,192			5,192
Other comprehensive income	155				155
Net loss	(14,124)					(14,124)
Ending balance at Mar. 31, 2019	$ 122,571		$ 1	$ 451,613	$ 73	$ (329,116)
Ending balance, shares at Mar. 31, 2019			45,157,652
[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

CONDENSED STATEMENTS OF CASH FL

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (14,124)	$ (15,493)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of intangible assets	36	36
Depreciation and amortization	617	376
Accretion of discounts on investments	(713)	(261)
Stock-based compensation expense	5,192	3,688
Changes in operating assets and liabilities
Accounts receivable	53	10,058
Prepaid expenses and other current assets	814	(1,122)
Other assets		(20)
Accounts payable	5,162	1,115
Accrued liabilities, income tax payable and other long-term liabilities	(6,605)	1,145
Deferred revenue	(29,461)	(14,134)
Net cash used in operating activities	(39,029)	(14,612)
Cash flows from investing activities:
Purchases of property and equipment	(1,835)	(751)
Purchases of short-term investments	(62,317)	(48,132)
Maturities of short-term investments	45,546	25,500
Net cash used in investing activities	(18,606)	(23,383)
Cash flows from financing activities:
Proceeds from exercise of stock options	465	2,602
Net cash provided by financing activities	465	2,602
Net decrease in cash, cash equivalents and restricted cash	(57,170)	(35,393)
Cash, cash equivalents and restricted cash, beginning of period	248,494	178,465
Cash, cash equivalents and restricted cash, end of period	191,324	143,072
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 245	$ 391

Description of the Business

Description of the Business	3 Months Ended
	Mar. 31, 2019
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 with a vision of transforming lives with safer, more effective therapeutics. The Company is pioneering a novel class of investigational antibody therapeutics, based on its Probody™ therapeutic technology platform for the treatment of cancer. The Probody therapeutic approach is designed to more specifically target antibody therapeutics to the tumor microenvironment and minimize drug activity in healthy tissue and in circulation. The Company is located in South San Francisco, California and was incorporated in the state of Delaware in September 2010.

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	2. Basis of Presentation and 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 condensed balance sheet data as of December 31, 2018 was derived from audited financial statements, but does not include all disclosures required by U.S. GAAP. The condensed results of operations for the three months ended March 31, 2019 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, 2018 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. 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. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that sum to the total of the amounts shown in the condensed statements of cash flows. March 31, 2019 December 31, 2018 March 31, 2018 December 31, 2017 (in thousands) Cash and cash equivalents $ 190,407 $ 247,577 $ 142,155 $ 177,548 Restricted cash - non-current assets 917 917 917 917 Total $ 191,324 $ 248,494 $ 143,072 $ 178,465 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 ’ ’ ’ The Company ’ The Company ’ The transaction price in each arrangement is allocated to the identified performance obligations based on the relative standalone selling price ( “ ” ’ In certain cases, the Company’s performance creates an asset that does not have an alternative use to the Company and the Company has an enforceable right to payment at all times for performance completed to date. In these cases, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. AbbVie Ireland Unlimited Company ( “ ” ’ “ ” ’ Contract Balances Customer payments are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the Company satisfies its performance obligations under the arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional. Research and Development Expenses 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 includes 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 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. In January, 2019, the Company acquired certain technology know-how that is complementary to the Company’s proprietary Probody technology, for $5.0 million. This technology will be utilized with certain of the Company’s discovery stage projects, and accordingly the $5.0 million is recorded as research and development expense for the three months ended March 31, 2019. Stock-based The Company recognizes compensation costs related to stock options granted to employees based on the The Company estimates the fair value of its stock-based awards using the Black-Scholes option-pricing • Expected term. The expected term of stock options represents the period that the stock options are expected to remain outstanding and is based on vesting terms, exercise term and contractual lives of the options. The expected term of the ESPP shares is equal to the six-month look-back period. • Expected volatility. The expected stock price volatility for the Company’s stock options was derived from the average historical volatilities of the Company’s stock price and the stock price of several comparable publicly traded companies within the biotechnology and pharmaceutical industry. The Company will continue to apply this process until a sufficient amount of historical information on the Company’s own stock price becomes available. • Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury whose term was consistent with expected term of the Company’s stock options. • Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and have no current plans to pay any dividends on our common stock. Leases The Company determines if an arrangement is or contains a lease at inception. Operating leases are recorded as operating lease right-of-use (“ROU”) assets and operating lease liabilities in the Company’s balance sheet. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses an implicit rate when readily available, or its incremental borrowing rate based on the information available at lease commencement date in determining the present value of lease payments. The operating lease ROU assets also include any lease prepayments made and reduced by lease incentives. The Company’s lease terms may include options to extend the lease when it is reasonably certain that such option will be exercised. Lease expenses are recognized on a straight-line basis over the lease term. The Company’s operating lease arrangement includes lease and non-lease components which are generally accounted for separately. See Note 9, Leases Adopted Accounting Pronouncements The Company adopted the Accounting Standard Update (“ASU”) No. 2016-02, Leases (Topic 842) n July 2018, further amended this standard to allow for a new transition method that provides the option to use the effective date as the date of initial application. The Company has elected such option and accordingly the comparative periods are not recast. The Company has also elected the package of practical expedients permitted under the new standard, or ASC 842. Accordingly, the Company continues to account for its existing operating leases as operating leases under the new guidance, without reassessing (a) whether the contracts contain a lease under ASC 842, (b) whether classification of the operating leases would be different in accordance with ASC 842, or (c) whether the unamortized initial direct costs before transition adjustments would have met the definition of initial direct costs in ASC 842 at lease commencement. In addition, the Company also elected the short-term lease practical expedients allowed under the standard. As a result of the adoption of ASC 842, the Company recorded a right-of-use asset of $28.0 million and a lease liability $30.1 million on January 1, 2019. This standard does not have material impact on the Company’s results of operations or cash flows. See Note 9, Leases. In March 2019, the FASB issued ASU No. 2019-01, Leases (Topic 842): Codification Improvements. Reclassification of Certain Tax Effects In February 2018, the FASB issued ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income Nonemployee Share-Based Payment In June 2018, the FASB issued ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Recent Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) In August 2018, the FASB issued ASU No. 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40) In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the interaction between Topic 808 and Topic 606

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	3. Net Loss Per Share Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period. Diluted net loss per share attributable to common stockholders is calculated using the weighted-average number of common shares outstanding, plus potential dilutive common stock during the period. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of the potentially dilutive securities is anti-dilutive. 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, 2019 2018 Options to purchase common stock 8,962,799 7,199,679 Total 8,962,799 7,199,679

Fair Value Measurements and Sho

Fair Value Measurements and Short-Term Investments	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements and Short-Term Investments	4. Fair Value Measurements and Short-Term Investments In accordance with Accounting Standards Codification (“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 assets which consist primarily of highly liquid money market funds, some of which are included in restricted cash, and U.S. government bonds that are included in short-term investments. The following tables set forth the fair value of the Company’s short-term investments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands): March 31, 2019 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 175,738 $ — $ — $ 175,738 Restricted cash (money market funds) Level I 917 — — 917 U.S. Government bonds Level I 206,058 131 — 206,189 Total $ 382,713 $ 131 $ — $ 382,844 As of March 31, 2019, no securities have contractual maturities of longer than one year. December 31, 2018 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 226,979 $ — $ — $ 226,979 Restricted cash (money market funds) Level I 917 — — 917 U.S. Government bonds Level I 188,616 — (66 ) 188,550 Total $ 416,512 $ — $ (66 ) $ 416,446

Accrued Liabilities

Accrued Liabilities	3 Months Ended
	Mar. 31, 2019
Accrued Liabilities Current [Abstract]
Accrued Liabilities	5. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): March 31, December 31, 2019 2018 (in thousands) Research and clinical expenses $ 14,613 $ 18,520 Payroll and related expenses 3,469 6,585 Legal and professional expenses 1,393 830 Operating lease liabilities - short term 2,544 - Other accrued expenses 548 789 Total $ 22,567 $ 26,724

Research and Collaboration Agre

Research and Collaboration Agreements	3 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Research and Collaboration Agreements	6. Research and Collaboration Agreements The following table summarizes the revenue by collaboration partners: Three Months Ended March 31, 2019 2018 (in thousands) AbbVie $ 2,948 $ 2,671 Amgen 1,035 1,255 BMS 25,502 8,168 ImmunoGen - 735 Pfizer - 1,355 Total Revenue $ 29,485 $ 14,184 AbbVie Ireland Unlimited Company In April 2016, the Company and AbbVie entered into two agreements, a CD71 Co-Development and Licensing Agreement (the “ ” “ ” “ ” “ ” 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, a 35% profit split on U.S. sales, and royalties on ex-U.S. sales in the high teens to low twenties if the Company participates in the co-development of the CD71 Licensed Product subject to a reversion to a royalty on U.S. sales, and reduction in royalties on ex-U.S. sales, if the Company opts-out from the co-development of the CD71 PDC. The Company ’ ’ “ ” Under the terms of the Discovery Agreement, AbbVie receives exclusive worldwide rights to develop and commercialize PDCs against up to two targets, one of which was selected in March 2017. The Company shall perform research services to discover the Probody therapeutics 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 ( “ ” 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 success criteria under the CD71 Agreement. AbbVie has not selected the second target, but the success criteria 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 Agreement and Discovery Agreement 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 performance obligations at the inception of the AbbVie Agreements: (1) the research, development and commercialization license for CD71 Probody therapeutic, (2) the research services related to CD71 Probody therapeutic, (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 ’ The Company determined that the research, development and commercialization licenses for CD71 and discovery targets are not distinct from the Company ’ ’ ’ ’ ’ (1) the CD71 Agreement performance obligation consisting of the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee, and (2) the Discovery Agreement performance obligation consisting of the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee. The total transaction price upon adoption of ASC 606 on January 1, 2018 of $39.8 million consists of $30.0 million in upfront payments, $14.0 million milestone payment received (net of the payment of an associated sublicense fee of $1.0 million to SGEN) less $4.2 million of estimated sublicense fees. The upfront payments under the AbbVie Agreements are allocated between the two performance obligations based on the estimated relative standalone selling prices. The $30.0 million of upfront payments is allocated $20.0 million to the CD71 Agreement, with the remaining $10.0 million allocated to the Discovery Agreement. The $14.0 million milestone payment received (net of the payment of an associated sublicense fee of $1.0 million to SGEN) and estimated sublicense fees of $4.2 million are allocated to the CD71 Agreement performance obligation as they are directly related to the development of the CD71 Probody therapeutic. In May 2018, the Company earned a $21.0 million milestone payment (net of the payment of an associated sublicense fee of $4.0 million to SGEN). The $21.0 million milestone payment was included as part of the transaction price in May 2018 and a revenue adjustment of $9.9 million was recognized in the second quarter of 2018 reflecting the percentage completed to-date on the project related to this milestone. The Company determined that the remaining potential milestone payments are probable of significant revenue reversal as their achievement is highly dependent on factors outside the Company ’ The Company is obligated to make sublicense payments under the sublicense agreement with the Regents of the University of California, acting through its Santa Barbara campus (“UCSB”), as amended, equal to 5% of certain upfront and milestone payments owed to or received by the Company. As of March 31, 2019 and December 31, 2018, the Company recorded a liability of $1.0 million and $1.1 million, respectively, representing the sublicense fee payable to UCSB under the CD71 Agreement. The $1.0 million liability on March 31, 2019 was paid in April 2019 after the Company and UCSB entered into Amendment No.3 to the Exclusive License Agreement. See Note 11, Subsequent Event. Of the $39.8 million total transaction price, the Company recognized the initial transaction price of $29.8 million allocated to the CD71 Agreement performance obligation using a cost-based input measure. In applying the cost-based input method, revenue is recognized based on actual full-time employee (“FTE”) hours incurred as a percentage of total estimated FTE hours as the Company completes its combined performance obligation over the five-year service period. As the Discovery Agreement performance obligation represents an obligation to continuously make the Company ’ The Company recognized revenue of $2.9 million and $2.7 million for the three months ended March 31, 2019 and 2018, respectively. related to the AbbVie Agreements. As of March 31, 2019 and December 31, 2018, deferred revenue related to the CD71 Agreement performance obligation was $20.7 million and $23.2 million, respectively, and deferred revenue related to the Discovery Agreement performance obligation was $4.2 million and $4.7 million, respectively. As of both March 31, 2019 and December 31, 2018, no amount was due from AbbVie under the CD71 and Discovery Agreements. Amgen, Inc. On September 29, 2017, the Company and Amgen, Inc. ( “ ” “ ” “ ” ’ Under the terms of the Amgen Agreement, the Company and Amgen will co-develop a Probody T-cell engaging bi-specific therapeutic targeting EGFR ( “ ” “ ” Amgen also has the right to select a total of up to three targets, including the two additional targets discussed below. The Company and Amgen collaborate in the research and development of Probody T-cell engaging bi-specifics products directed against such targets. Amgen has selected one such target (the “ ” “ ” “ ” ’ At the initiation of the collaboration, CytomX had the option to select, from programs specified in the Amgen Agreement, an existing pre-clinical stage T-cell engaging bispecific product from the Amgen pre-clinical pipeline. In March 2018, CytomX selected the program. CytomX is responsible, at its expense, for converting this program to a Probody T-cell engaging bispecific product, and thereafter, will be responsible for development, manufacturing, and commercialization of the product ( “ ” The Company considered the criteria for combining contracts in ASC 606 and determined that the Amgen Agreement and the Purchase Agreement should be combined into one contract. The Company accounted for the Amgen Agreement based on the fair values of the assets and services exchanged. The Company identified the following performance obligations at the inception of the Amgen Agreement: (1) the research, development and commercialization license, (2) the research and development services for the EGFR Products and the Amgen Other Product, and (3) the obligation to participate in the joint steering committee (“JSC”) and the joint research committee (“JRC”). The Company determined that research, development and commercialization license and the participation in the JSC and JRC are not distinct from the research and development services and therefore those performance obligations were combined into one combined performance obligation. The Amgen Other Products are accounted for as a separate performance obligation from the EGFR Products as the nature of the services being performed is not the same and the value that Amgen can derive from one program is not dependent on the success of the other. Concurrent with the execution of the Amgen Agreement, the Company entered into a sublicense agreement whereby the Company granted Amgen a sublicense of its rights to one patent family that it co-owns with UCSB, that is exclusively licensed to the Company under the UCSB Agreement covering Probody antibodies and other pro-proteins in the fields of therapeutics, in vivo diagnostics and prophylactics. This sublicense was incremental to the patents, patent applications and know-how covering T-cell engaging bispecific Probody molecules that were developed and owned by the Company and licensed to Amgen. Under the UCSB Agreement, as amended, the Company is obligated to make a sublicense payment to UCSB equal to up to 7.5% of certain upfront and milestone payments owed to or received by the Company. As of March 31, 2019 and December 31, 2018, the Company recorded a liability of $1.9 million and $2.1 million, respectively, representing the sublicense fee payable to UCSB. The $1.9 million liability on March 31, 2019 was paid in April 2019 after the Company and UCSB entered into the Amendment No.3 to the Exclusive License Agreement. See Note 11, Subsequent Event. The total transaction price of $51.2 million, consisting of the $40.0 million upfront payment, an estimated fair value of $10.7 million for the CytomX Product and $0.5 million of premium on the sale of the Company ’ ’ Of the $51.2 million total transaction price, the Company allocated $46.4 million to the EGFR Products performance obligation and $4.8 million to the Amgen Other Product performance obligations. The transaction price of the EGFR Product performance obligation was recognized using a cost-based input measure. In applying the cost-based input method of revenue recognition, the Company uses actual FTE hours incurred relative to estimated total FTE hours expected to be incurred for the combined performance obligation over the research service period. In the fourth quarter of 2018, the JSC discontinued work on two molecules that did not meet required research criteria. Pre-clinical evaluation of additional molecules is ongoing as part of the candidate identification phase of the EGFR project, and as a result, there was a change in estimate of the projected costs and research service period to seven years during the fourth quarter of 2018. The $4.8 million transaction price allocated to the Amgen Other Product performance obligation represents an obligation to continuously make the Probody therapeutic technology platform available to Amgen, which is recognized over the common measure of progress for the entire performance obligation over the estimated research service period of six years. The Company recognized revenue of $1.0 million and $1.3 million for the three months ended March 31, 2019 and 2018, respectively, related to the Amgen Agreement. As of March 31, 2019 and December 31, 2018, deferred revenue related to the EGFR Products performance obligation was $39.8 million and $40.7 million, respectively. As of March 31, 2019 and December 31, 2018 deferred revenue related to the Amgen Other Products performance obligation was $3.6 million and $3.8 million, respectively. As of March 31, 2019 and December 31 2018, no amount was due from Amgen under the Amgen Agreement. Bristol-Myers Squibb Company On May 23, 2014, the Company and Bristol-Myers Squibb Company ( “ ” “ ” ’ 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 had additional rights to substitute up to two collaboration targets within three years of the effective date of the BMS Agreement. These rights expired in May 2017. Each collaboration target had a two-year research term and the two additional targets had to be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target could 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 the Company 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-single digits to low double-digit percentages from potential future sales. The Company also receives research and development service fees based on a prescribed FTE rate that is capped. The Company identified the following performance obligations 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, the Company ’ ’ On March 17, 2017, the Company and BMS entered into Amendment Number 1 to Extend Collaboration and License Agreement (the “ ” “ ” Pursuant to the Amendment, the financial consideration from BMS is comprised of an upfront payment of $200.0 million and the Company is eligible to receive up to an aggregate of $3,586.0 million as follows: (i) up to $116.0 million in development milestone payments per target or up to $928.0 million if the maximum of eight targets are selected for the first product modality; (ii) up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per target program or up to $992.0 million if the maximum of eight targets are selected for the first product modality; (iii) up to $60.0 million in sales milestone payments per target or up to $480.0 million if maximum of eight targets are selected for the first product modality; and (iv) up to $56.3 million in development milestone payments or up to $450.0 million if the maximum of eight targets are selected for the second product modality; (v) up to $62.0 million in milestone payments for the first commercial sale in various territories for up to three indications per target program or up to $496.0 million if the maximum of eight targets are selected for the second product modality; (vi) up to $30.0 million in sales milestone payments per target or up to $240.0 million if maximum of eight targets are selected for the second product modality. The Company is also entitled to tiered mid-single to low double-digit percentage royalties from potential future sales. The Amendment does not change the term of the BMS ’ ’ The initial transaction price is $272.8 million consisting of the upfront fees of $250.0 million, research and development service fees of $10.8 million and milestone payments received to date of $12.0 million. The Company determined that the remaining potential milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company ’ During the first quarter of 2019, BMS terminated pre-clinical activities on three of the first four collaboration targets selected under the BMS Agreement. The first and second targets under the BMS Agreement were combined into a single performance obligation. The Company determined that termination of pre-clinical activities on the second target does not impact the Company’s continuing obligation to BMS for the first target, CTLA-4, as it is still being actively developed by BMS. Therefore, the Company concluded that it will continue to amortize the related deferred revenue over the original performance period. The Company has determined that upon the termination of pre-clinical activities on the third and the fourth collaboration targets selected by BMS in January and December of 2016, respectively, under the BMS Agreement, it has no further obligations and is no longer eligible to receive any further proceeds from milestones, royalties or research and development fees for such targets. As a result, the Company accelerated recognition of all of the related deferred revenue of the third and the fourth targets upon the effective date of termination and recognized $17.4 million in the first quarter of 2019. The Company recognized revenue of $25.5 million and $8.2 million for the three months ended March 31, 2019 and 2018, respectively. As of March 31, 2019 and December 31, 2018, deferred revenue relating to the BMS Agreement was $180.2 million and $205.6 million, respectively. The amount due from BMS under the BMS Agreement was $44,000 and $0.1 million as of March 31, 2019 and December 31, 2018, respectively. ImmunoGen, Inc. In January 2014, the Company and ImmunoGen, Inc. ( “ ” “ ” ’ “ ” ’ ’ ’ “ ” “ ” Under the terms of the ImmunoGen Research Agreement, both the Company and ImmunoGen performed research activities on behalf of the other party for no monetary consideration through January 2018 and the arrangement was extended to June 2018, as discussed below. 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. In consideration for the ImmunoGen 2017 License, the Company is entitled to receive up to $30.0 million in development and regulatory milestone payments, up to $50.0 million in sales milestone payments and royalties in the mid-single digit percentage on the commercial sales of any resulting product. For the CX-2009 License, the Company is obligated to pay ImmunoGen up to $60.0 million in development and regulatory milestone payments and up to $100.0 million in sales milestone payments and royalties in the mid to high single digits percentage on the commercial sales of any resulting product. In August 2017, the Company made a milestone payment of $1.0 million to ImmunoGen for the first patient dosing with CX-2009. No milestone payments have been accrued to the Company under the ImmunoGen 2017 License. The Company accounted for the ImmunoGen Research Agreement based on the fair value of the assets and services exchanged. The Company identified the following performance obligations at the inception of the ImmunoGen Research 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, the research services, the participation in the joint steering committee, and the technology improvements are not distinct from the development and commercialization license and therefore those performance obligations were combined into one combined performance obligation. The Company considered factors such as the limited economic benefits to ImmunoGen if the development and commercialization license was 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 at the inception of the ImmunoGen Research Agreement. In December 2017, the Company entered into the ImmunoGen 2017 License arrangement and extended the Company ’ The estimated total 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 allocated to the research services, joint research committee participation and technology improvements, which was expensed over the period of services provided. The Company recognized revenue of $0 and $0.7 million for this target for the three months ended March 31, 2019 and 2018, respectively. As of March 31, 2019 and December 31, 2018, there was no deferred revenue relating to the ImmunoGen Research Agreement. As of both March 31, 2019 and December 31, 2018, no amount was due from ImmunoGen under the ImmunoGen Research Agreement. Pfizer Inc. In May 2013, the Company and Pfizer Inc. ( “ ” “ ” “ ” Pursuant to the Pfizer Agreement, the Company received an upfront payment of $6.0 million and research and development service fees based on a prescribed FTE rate per year that is capped. The Company identified the following performance obligations 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 was not distinct from the research services and participation in the joint research committee 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 combined performance obligation. The Company concluded that, at the inception of the agreement, Pfizer ’ As the combined performance obligation represented an obligation to continuously make the Probody therapeutic technology platform available to Pfizer, the initial transaction price was recognized over the common measure of progress for the entire performance obligation over the estimated research service period of five and a half years. The Company recognized revenue of $0 and $1.4 million for the three months ended March 31, 2019 and 2018, respectively. As of March 31, 2019 and December 31, 2018, there was no deferred revenue relating to the Pfizer Agreement. As of both March 31, 2019 and December 31, 2018, the amount due from Pfizer under the Pfizer Agreement was $0. Contract Liabilities The following table presents changes in the Company’s total contract liabilities during the three months ended March 31, 2019: Balance at Beginning of Period Additions Deductions Balance at End of Period (in thousands) Contract liabilities: Deferred revenue $ 277,980 $ — $ (29,461 ) $ 248,519 There were no additions to deferred revenue during the first quarter of 2019. Deductions of $29.5 million represents revenue recognized, including the accelerated recognition of deferred revenue of $17.4 million due to termination of certain targets by BMS, in the three months ended March 31, 2019 that was included in the contract liability balance at the beginning of the period. The Company estimates that the $248.5 million of deferred revenue related to the following contracts as of March 31, 2019 to be recognized as revenue as set forth below. However, the timing of revenue recognition could differ from the estimates depending on facts and circumstances impacting the various contracts, including progress of research and development, resources assigned to the contracts by the Company or its collaboration partners, or other factors outside of the Company’s control. • The $20.7 million of deferred revenue related to the CD71 Agreement as of March 31, 2019 is expected to be recognized based on actual FTE effort and program progress until approximately April 2021. • The $4.2 million of deferred revenue related to the Discovery Agreement as of March 31, 2019 is expected to be recognized ratably until approximately April 2021. • The $39.8 million of deferred revenue related to the Amgen EGFR Products as of March 31, 2019 is expected to be recognized based on actual FTE effort and program progress until approximately September 2024. • The $3.6 million of deferred revenue related to the Amgen Other Products as of March 31, 2019 is expected to be recognized ratably until approximately September 2023. • The $180.2 million of deferred revenue related to the BMS Agreement as of March 31, 2019 is expected to be recognized ratably until approximately April 2025.

UCSB License Agreement

UCSB License Agreement	3 Months Ended
	Mar. 31, 2019
Research And Development [Abstract]
UCSB License Agreement	7. UCSB License Agreement The Company has an exclusive, worldwide license agreement with UCSB (the “ ” Pursuant to the UCSB Agreement, the Company is obligated to (i) make royalty payments to UCSB on net sales of its products covered under the agreement, subject to annual minimum amounts, (ii) make milestone payments to UCSB upon the occurrence of certain events, (iii) make a milestone payment to UCSB upon occurrence of an IPO or change of control, and (iv) reimburse UCSB for prosecution and maintenance of the licensed patents. If the Company sublicenses its rights under the UCSB Agreement, it is obligated to pay UCSB a percentage of the total sublicense revenue received, which total amount would be first reduced by the aggregate amount of certain research and development related expenses incurred by the Company and other permitted deductions. As part of the UCSB Agreement, the Company has annual minimum royalty obligations of $150,000 under the terms of certain exclusive licensed patent rights. The royalty obligations are cancellable any time by giving notice to the licensor, with the termination being effective 60 days after giving notice. In 2013, the Company amended the UCSB Agreement to reduce certain amounts due to UCSB upon receipt by the Company of upfront payments, milestone payments and royalties from sublicensees. In exchange for this amendment, the Company issued to UCSB 157,332 shares of common stock. The UCSB Agreement, as amended, will remain in effect until the expiration or abandonment of the last to expire of the licensed patents. During the three months ended March 31, 2019 and 2018, the Company did not incur any material sublicense fee under the provisions of the UCSB Agreement. In April 2019, the Company entered into Amendment No.3 to the UCSB Agreement (the “Amendment No.3”) with UCSB to, among other things, clarify and adjust the sublicense terms of this UCSB Agreement. As a result of the Amendment No.3, the Company is required to pay an additional annual maintenance fee of $750,000 from April 2019 through 2031. See Note 11, Subsequent Event.

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation Stock Options Activities under the Company’s stock option plans for the three months ended March 31, 2019 were as follows: Options Outstanding Number of Options Weighted- Average Exercise Price Per Share Balances at December 31, 2018 7,803,773 $ 12.622 Options granted 2,103,883 14.736 Options exercised (74,443 ) 6.240 Option forfeited/expired (61,025 ) 17.894 Balances at March 31, 2019 9,772,188 13.093 Options exercisable at March 31, 2019 4,854,438 $ 8.558 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: Three Months Ended March 31, 2019 2018 (in thousands) Stock-based compensation expense: Research and development $ 2,642 $ 1,808 General and administrative 2,550 1,880 Total stock-based compensation expense $ 5,192 $ 3,688

Leases

Leases	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Leases	9. Leases Operating Lease On December 10, 2015, the Company entered into a lease (the “2016 Lease”) with HCP Oyster Point III LLC (the “Landlord”) to lease approximately 76,000 rentable square feet of office and laboratory space located in South San Francisco, California for the Company’s new corporate headquarters. The term of the Lease commenced on October 1, 2016. The 2016 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 2016 Lease. The Lease provided 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 was 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 2016 Lease. The Company has recorded the $0.9 million Letter of Credit in restricted cash as non-current on its balance sheet at March 31, 2019 and December 31, 2018, respectively. 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 against the operating lease ROU asset. Rent expense for both the three months ended March 31, 2019 and 2018 was $1.3 million. Supplemental information related to leases are as follows: Three Months Ended March 31, 2019 (in thousands) Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 1,205 March 31, 2019 (in thousands) Supplemental balance sheet information related to leases: Operating lease right-of-use assets $ 27,404 Current operating lease liabilities 2,544 Non-current operating lease liabilities 27,008 Total operating lease liabilities $ 29,552 Weighted-average remaining lease term (in years) Operating lease 7.61 Weighted-average discount rate Operating lease 8.25 % March 31, 2019 (in thousands) Maturity of operating lease liabilities 2019 $ 4,888 2020 5,025 2021 5,165 2022 5,309 2023 and beyond 19,763 Total lease payments 40,150 Less imputed interest (10,598 ) Present value of lease liabilities $ 29,552

Income Tax Expense

Income Tax Expense	3 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income Tax Expense	10. Income Tax Expense The Company recorded an income tax benefit of $6,000 and income tax expense of $1.1 million during the three months ended March 31, 2019 and 2018, respectively. The income tax benefit for the three months ended March 31, 2019 was as a result of an unrealized gain on the available for sale securities recorded in other comprehensive income during the period. The income tax expense during the three months ended March 31, 2018 was generated as a result of a temporary difference in the recognition of revenue under tax and U.S. GAAP authoritative guidance, primarily due to revenue recognition for tax purposes in 2018 of the upfront payments received pursuant to the BMS Amendment entered into in March 2017. The Company’s effective tax rate was 0.04% and (7.63)% for the three months ended March 31, 2019 and 2018, respectively. The Company maintains a full valuation allowance against its net deferred tax assets due to the Company’s history of losses as of March 31, 2019 .

Subsequent Event

Subsequent Event	3 Months Ended
	Mar. 31, 2019
Subsequent Events [Abstract]
Subsequent Event	11. Subsequent Event On April 2, 2019, the Company, entered into Amendment No.3 to the UCSB Agreement with UCSB to adjust and clarify certain sublicense terms (the “Amendment No.3”). In connection with the entry into the Amendment No.3, the Company issued to UCSB 150,000 shares of CytomX common stock, pursuant to a Securities Issuance Agreement, dated April 2, 2019, by and between CytomX and UCSB. Under the terms of the Amendment No.3, the Company and UCSB agreed to modify the Company’s obligation to pay UCSB a percentage of sublicense revenues to be earned in the future on existing and new collaboration agreements. In particular, the Amendment No.3 adjusts the definition of sublicense revenues, which includes up-front fees, sublicense maintenance fees, milestone payments or other sublicense revenues (other than royalties) and modifies the Company’s payment obligation for pre-clinical stage sublicenses going forward by potentially lowering the sublicense revenue payment to UCSB based on the contribution of intellectual property from each party. In exchange, the Company agreed to make an upfront payment of $1,000,000 as well as additional annual license maintenance fees of $750,000 through 2031.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
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 condensed balance sheet data as of December 31, 2018 was derived from audited financial statements, but does not include all disclosures required by U.S. GAAP. The condensed results of operations for the three months ended March 31, 2019 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, 2018 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.
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. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that sum to the total of the amounts shown in the condensed statements of cash flows. March 31, 2019 December 31, 2018 March 31, 2018 December 31, 2017 (in thousands) Cash and cash equivalents $ 190,407 $ 247,577 $ 142,155 $ 177,548 Restricted cash - non-current assets 917 917 917 917 Total $ 191,324 $ 248,494 $ 143,072 $ 178,465
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 ’ ’ ’ The Company ’ The Company ’ The transaction price in each arrangement is allocated to the identified performance obligations based on the relative standalone selling price ( “ ” ’ In certain cases, the Company’s performance creates an asset that does not have an alternative use to the Company and the Company has an enforceable right to payment at all times for performance completed to date. In these cases, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. AbbVie Ireland Unlimited Company ( “ ” ’ “ ” ’
Contract Balances	Contract Balances Customer payments are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the Company satisfies its performance obligations under the arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional.
Research and Development Expenses	Research and Development Expenses 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 includes 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 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. In January, 2019, the Company acquired certain technology know-how that is complementary to the Company’s proprietary Probody technology, for $5.0 million. This technology will be utilized with certain of the Company’s discovery stage projects, and accordingly the $5.0 million is recorded as research and development expense for the three months ended March 31, 2019.
Stock-based Compensation	Stock-based The Company recognizes compensation costs related to stock options granted to employees based on the The Company estimates the fair value of its stock-based awards using the Black-Scholes option-pricing • Expected term. The expected term of stock options represents the period that the stock options are expected to remain outstanding and is based on vesting terms, exercise term and contractual lives of the options. The expected term of the ESPP shares is equal to the six-month look-back period. • Expected volatility. The expected stock price volatility for the Company’s stock options was derived from the average historical volatilities of the Company’s stock price and the stock price of several comparable publicly traded companies within the biotechnology and pharmaceutical industry. The Company will continue to apply this process until a sufficient amount of historical information on the Company’s own stock price becomes available. • Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury whose term was consistent with expected term of the Company’s stock options. • Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and have no current plans to pay any dividends on our common stock.
Leases	Leases The Company determines if an arrangement is or contains a lease at inception. Operating leases are recorded as operating lease right-of-use (“ROU”) assets and operating lease liabilities in the Company’s balance sheet. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses an implicit rate when readily available, or its incremental borrowing rate based on the information available at lease commencement date in determining the present value of lease payments. The operating lease ROU assets also include any lease prepayments made and reduced by lease incentives. The Company’s lease terms may include options to extend the lease when it is reasonably certain that such option will be exercised. Lease expenses are recognized on a straight-line basis over the lease term. The Company’s operating lease arrangement includes lease and non-lease components which are generally accounted for separately. See Note 9, Leases
Adopted Accounting Pronouncements and Recent Accounting Pronouncements	Adopted Accounting Pronouncements The Company adopted the Accounting Standard Update (“ASU”) No. 2016-02, Leases (Topic 842) n July 2018, further amended this standard to allow for a new transition method that provides the option to use the effective date as the date of initial application. The Company has elected such option and accordingly the comparative periods are not recast. The Company has also elected the package of practical expedients permitted under the new standard, or ASC 842. Accordingly, the Company continues to account for its existing operating leases as operating leases under the new guidance, without reassessing (a) whether the contracts contain a lease under ASC 842, (b) whether classification of the operating leases would be different in accordance with ASC 842, or (c) whether the unamortized initial direct costs before transition adjustments would have met the definition of initial direct costs in ASC 842 at lease commencement. In addition, the Company also elected the short-term lease practical expedients allowed under the standard. As a result of the adoption of ASC 842, the Company recorded a right-of-use asset of $28.0 million and a lease liability $30.1 million on January 1, 2019. This standard does not have material impact on the Company’s results of operations or cash flows. See Note 9, Leases. In March 2019, the FASB issued ASU No. 2019-01, Leases (Topic 842): Codification Improvements. Reclassification of Certain Tax Effects In February 2018, the FASB issued ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income Nonemployee Share-Based Payment In June 2018, the FASB issued ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Recent Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) In August 2018, the FASB issued ASU No. 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40) In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the interaction between Topic 808 and Topic 606

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Schedule of Cash, Cash Equivalents, and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that sum to the total of the amounts shown in the condensed statements of cash flows. March 31, 2019 December 31, 2018 March 31, 2018 December 31, 2017 (in thousands) Cash and cash equivalents $ 190,407 $ 247,577 $ 142,155 $ 177,548 Restricted cash - non-current assets 917 917 917 917 Total $ 191,324 $ 248,494 $ 143,072 $ 178,465

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2019
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, 2019 2018 Options to purchase common stock 8,962,799 7,199,679 Total 8,962,799 7,199,679

Fair Value Measurements and S_2

Fair Value Measurements and Short-Term Investments (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis	The following tables set forth the fair value of the Company’s short-term investments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands): March 31, 2019 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 175,738 $ — $ — $ 175,738 Restricted cash (money market funds) Level I 917 — — 917 U.S. Government bonds Level I 206,058 131 — 206,189 Total $ 382,713 $ 131 $ — $ 382,844 December 31, 2018 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 226,979 $ — $ — $ 226,979 Restricted cash (money market funds) Level I 917 — — 917 U.S. Government bonds Level I 188,616 — (66 ) 188,550 Total $ 416,512 $ — $ (66 ) $ 416,446

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	3 Months Ended
	Mar. 31, 2019
Accrued Liabilities Current [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): March 31, December 31, 2019 2018 (in thousands) Research and clinical expenses $ 14,613 $ 18,520 Payroll and related expenses 3,469 6,585 Legal and professional expenses 1,393 830 Operating lease liabilities - short term 2,544 - Other accrued expenses 548 789 Total $ 22,567 $ 26,724

Research and Collaboration Ag_2

Research and Collaboration Agreements (Tables)	3 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Revenue by Collaboration Partners	The following table summarizes the revenue by collaboration partners: Three Months Ended March 31, 2019 2018 (in thousands) AbbVie $ 2,948 $ 2,671 Amgen 1,035 1,255 BMS 25,502 8,168 ImmunoGen - 735 Pfizer - 1,355 Total Revenue $ 29,485 $ 14,184
Summary of Contract Liabilities	The following table presents changes in the Company’s total contract liabilities during the three months ended March 31, 2019: Balance at Beginning of Period Additions Deductions Balance at End of Period (in thousands) Contract liabilities: Deferred revenue $ 277,980 $ — $ (29,461 ) $ 248,519

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Activities Under Company's Stock Option Plans	Activities under the Company’s stock option plans for the three months ended March 31, 2019 were as follows: Options Outstanding Number of Options Weighted- Average Exercise Price Per Share Balances at December 31, 2018 7,803,773 $ 12.622 Options granted 2,103,883 14.736 Options exercised (74,443 ) 6.240 Option forfeited/expired (61,025 ) 17.894 Balances at March 31, 2019 9,772,188 13.093 Options exercisable at March 31, 2019 4,854,438 $ 8.558
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: Three Months Ended March 31, 2019 2018 (in thousands) Stock-based compensation expense: Research and development $ 2,642 $ 1,808 General and administrative 2,550 1,880 Total stock-based compensation expense $ 5,192 $ 3,688

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Summary of Supplemental Information Related to Leases	Supplemental information related to leases are as follows: Three Months Ended March 31, 2019 (in thousands) Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 1,205 March 31, 2019 (in thousands) Supplemental balance sheet information related to leases: Operating lease right-of-use assets $ 27,404 Current operating lease liabilities 2,544 Non-current operating lease liabilities 27,008 Total operating lease liabilities $ 29,552 Weighted-average remaining lease term (in years) Operating lease 7.61 Weighted-average discount rate Operating lease 8.25 % March 31, 2019 (in thousands) Maturity of operating lease liabilities 2019 $ 4,888 2020 5,025 2021 5,165 2022 5,309 2023 and beyond 19,763 Total lease payments 40,150 Less imputed interest (10,598 ) Present value of lease liabilities $ 29,552

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018		Mar. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 190,407	$ 247,577	[1]	$ 142,155	$ 177,548
Restricted cash	917	917	[1]	917	917
Total	$ 191,324	$ 248,494		$ 143,072	$ 178,465
[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2019	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019
Summary Of Significant Accounting Policies [Line Items]
Research and development		$ 36,376	$ 22,458
Right-of-use asset		27,404
Lease liability		29,552
ASC 842
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset				$ 28,000
Lease liability				$ 30,100
ASU 2018-02
Summary Of Significant Accounting Policies [Line Items]
Increase in other comprehensive income		11,000
Probody technology
Summary Of Significant Accounting Policies [Line Items]
Acquired, technology know-how	$ 5,000
Research and development		$ 5,000

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, 2019	Mar. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	8,962,799	7,199,679
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	8,962,799	7,199,679

Fair Value Measurements and S_3

Fair Value Measurements and Short-Term Investments - Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 382,713	$ 416,512
Gross Unrealized Holding Gains	131
Gross Unrealized Holding Losses		(66)
Aggregate Fair Value	382,844	416,446
Level I | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	175,738	226,979
Aggregate Fair Value	175,738	226,979
Level I | Restricted cash (money market funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	917	917
Aggregate Fair Value	917	917
Level I | U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	206,058	188,616
Gross Unrealized Holding Gains	131
Gross Unrealized Holding Losses		(66)
Aggregate Fair Value	$ 206,189	$ 188,550

Fair Value Measurements and S_4

Fair Value Measurements and Short-Term Investments - Additional Information (Details)	Mar. 31, 2019USD ($)
Fair Value Disclosures [Abstract]
Contractual maturities of longer than one year	$ 0

Accrued Liabilities - Summary o

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Research and clinical expenses	$ 14,613	$ 18,520
Payroll and related expenses	3,469	6,585
Legal and professional expenses	1,393	830
Operating lease liabilities - short term	2,544
Other accrued expenses	548	789
Total	$ 22,567	$ 26,724	[1]
[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

Research and Collaboration Ag_3

Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partners (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 29,485	$ 14,184
AbbVie
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	2,948	2,671
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	1,035	1,255
BMS
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 25,502	8,168
ImmunoGen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		735
Pfizer
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		$ 1,355

Research and Collaboration Ag_4

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)	Apr. 02, 2019USD ($)	May 31, 2018USD ($)	Jul. 31, 2017USD ($)	Apr. 30, 2016USD ($)AgreementTargetAccountingUnit	Mar. 31, 2019USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)		Mar. 31, 2017Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities					$ 22,567,000			$ 26,724,000	[1]
Revenue recognized from collaborative arrangement					$ 29,485,000		$ 14,184,000
Type of Revenue [Extensible List]					us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember
Deferred revenue					$ 248,519,000			277,980,000
Seattle Genetics Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid		$ 4,000,000
AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment received		21,000,000
Revenue adjustment of milestone payment recognized						$ 9,900,000
Revenue recognized from collaborative arrangement					$ 2,900,000		$ 2,700,000
Type of Revenue [Extensible List]					us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember
AbbVie Ireland Unlimited Company | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received				$ 30,000,000
Milestone payment received				14,000,000
Total transaction price				39,800,000
Estimated sublicense fees				$ 4,200,000
Number of accounting units | AccountingUnit				2
AbbVie Ireland Unlimited Company | 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%
Amount due from agreement					$ 0			0
AbbVie Ireland Unlimited Company | CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received				$ 20,000,000
Milestone payment received		21,000,000	$ 14,000,000
Sublicense fees paid			$ 1,000,000
Deferred revenue					20,700,000			23,200,000
AbbVie Ireland Unlimited Company | CD71 Agreement | UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities					$ 1,000,000			1,100,000
AbbVie Ireland Unlimited Company | CD71 Agreement | UC Regents | Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments for sublicense fee payable	$ 1,000,000
AbbVie Ireland Unlimited Company | CD71 Agreement | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received				20,000,000
Total transaction price				29,800,000
Estimated sublicense fees				4,200,000
Milestone payment received				$ 14,000,000
AbbVie Ireland Unlimited Company | CD71 Agreement | U.S
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Profit split on U.S. sales and royalties on ex U.S. sales				35.00%
AbbVie Ireland Unlimited Company | CD71 Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development, regulatory and commercial milestones				$ 470,000,000
AbbVie Ireland Unlimited Company | Seattle Genetics Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid		$ 4,000,000
AbbVie Ireland Unlimited Company | Seattle Genetics Agreement | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid				1,000,000
AbbVie Ireland Unlimited Company | Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received				10,000,000
Number of targets selected | Target										1
Revenue recognition upon performance obligation				10,000,000
Estimated research service period					5 years
Deferred revenue					$ 4,200,000			$ 4,700,000
AbbVie Ireland Unlimited Company | Discovery Agreement | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received				$ 10,000,000
AbbVie Ireland Unlimited Company | Discovery Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets | Target				2
AbbVie Ireland Unlimited Company | Discovery Agreement | Maximum | Development, Regulatory and Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable				$ 275,000,000
AbbVie Ireland Unlimited Company | Sublicense Agreement | UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense payment percentage				5.00%
[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

Research and Collaboration Ag_5

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details 1)	Mar. 31, 2019
AbbVie Ireland Unlimited Company | CD71 Agreement | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-04-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognition upon performance obligation, service period	5 years

Research and Collaboration Ag_6

Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)	Apr. 02, 2019USD ($)	Sep. 29, 2017USD ($)Target	Oct. 31, 2017USD ($)$ / sharesshares	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)		Mar. 31, 2018USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities				$ 22,567,000	$ 26,724,000	[1]
Revenue recognized from collaborative arrangement				$ 29,485,000			$ 14,184,000
Type of Revenue [Extensible List]				us-gaap:LicenseAndServiceMember			us-gaap:LicenseAndServiceMember
Deferred revenue				$ 248,519,000	$ 277,980,000
Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				39,800,000
Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				3,600,000
Collaboration and License Agreement | Amgen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			$ 40,000,000
Common stock, shares issuable under agreement | shares			1,156,069
Common stock, shares issuable under agreement, price per share | $ / shares			$ 17.30
Common stock, value of shares issued in connection with agreement			$ 20,000,000
Period used to calculate weighted average price per share			20 days
Lock-up period for share disposal			6 months
Estimated premium on the stock sold			$ 500,000
Number of targets selected | Target		1
Number of additional collaboration target | Target		2
Total transaction price		$ 51,200,000
Portion of transaction price allocated to premium on sale of common stock		500,000
Estimated fair value of products		10,700,000
Revenue recognized from collaborative arrangement				$ 1,000,000			$ 1,300,000
Type of Revenue [Extensible List]				us-gaap:LicenseAndServiceMember			us-gaap:LicenseAndServiceMember
Amount due from agreement				$ 0			$ 0
Collaboration and License Agreement | Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price				46,400,000
Estimated projected costs and research service period					7 years
Deferred revenue				39,800,000	$ 40,700,000
Collaboration and License Agreement | Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price				4,800,000
Revenue recognition upon performance obligation				$ 4,800,000
Estimated research service period				6 years
Deferred revenue				$ 3,600,000	3,800,000
Collaboration and License Agreement | Amgen Inc | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 950,000,000
Number of targets | Target		3
Contingent payments payable		$ 203,000,000
Collaboration and License Agreement | Amgen Inc | Maximum | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 455,000
Percentage share of profit and losses		50.00%
Sublicense Agreement | Amgen Inc | UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities				$ 1,900,000	$ 2,100,000
Sublicense Agreement | Amgen Inc | UC Regents | Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments for sublicense fee payable	$ 1,900,000
Sublicense Agreement | Amgen Inc | Maximum | UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense payment percentage		7.50%
[1]	The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.

Research and Collaboration Ag_7

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)	Apr. 25, 2017USD ($)TargetSaleIndicator	Jul. 07, 2014USD ($)TargetTermResearchTargetSaleIndicator	May 23, 2014USD ($)	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)	Nov. 30, 2017USD ($)	Mar. 17, 2017Target	Dec. 31, 2016USD ($)	Jan. 31, 2016USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement				$ 29,485,000	$ 14,184,000
Type of Revenue [Extensible List]				us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue				$ 248,519,000		$ 277,980,000
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				$ 180,200,000
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				Each collaboration target had a two-year research term and the two additional targets had to be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target could be extended in one year increments up to three times.
Extension of research term for each collaboration target		1 year
Upfront payment received	$ 200,000,000	$ 50,000,000	$ 250,000,000
Contingent milestone payments receivable									$ 15,000,000	$ 10,000,000
Revenue recognition upon performance obligation			272,800,000
Upfront payment received			10,800,000
Milestone payment received			$ 12,000,000
Estimated research service termination date				Apr. 25, 2025
Revenue recognized from collaborative arrangement				$ 25,500,000	$ 8,200,000
Type of Revenue [Extensible List]				us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue				$ 180,200,000		205,600,000
Amount due from agreement				44,000,000,000		$ 100,000
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 | New Drug Application for CTLA-4
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable							$ 10,000,000
Collaborative Arrangement | Bristol Myers Squibb Company | Third And Fourth Immuno-Oncology Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement				$ 17,400,000
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
Aggregate future contingent milestone payment		$ 1,217,000,000							$ 3,586,000,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Each Of Two Additional Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment		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]
Aggregate future contingent milestone payment	116,000,000	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]
Aggregate future contingent milestone payment		$ 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]
Aggregate future contingent milestone payment	$ 124,000,000	$ 124,000,000
Number of sales indicators | SaleIndicator	3	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]
Aggregate future contingent milestone payment		$ 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]
Aggregate future contingent milestone payment	$ 60,000,000	$ 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]
Aggregate future contingent milestone payment		$ 240,000,000
Number of research targets selected | ResearchTarget		4
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achievement of Development Milestones if Eight Research Targets Selected by Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 928,000,000
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Eight Targets Selected for the First Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 992,000,000
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestones if Eight Research Targets Selected for First Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 480,000,000
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Development Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 56,300,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Development Milestone Payment if Eight Targets are Selected for Second Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 450,000,000
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Milestone Payments for First Commercial Sale in Various Territories for Up to Three Indication
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 62,000,000
Number of sales indicators | SaleIndicator	3
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Development Milestone Payment if Eight Targets are Selected for Second Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 496,000,000
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 30,000,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestone Payment if Eight Research Target are selected for Second Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 240,000,000
Number of research targets selected | Target	8
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 | Maximum | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target | Target								6

Research and Collaboration Ag_8

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended
	Aug. 31, 2017	Jan. 31, 2014	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue			$ 248,519,000		$ 277,980,000
Revenue recognized from collaborative arrangement			$ 29,485,000	$ 14,184,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Collaborative Arrangement | Immuno Gen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment		$ 0
Milestones payments made during period	$ 1,000,000
Milestone payments accrued			$ 0
Estimated total fair value consideration recorded as deferred revenue		13,200,000	0		0
Estimated fair value of assets and services		13,200,000
Revenue recognized from collaborative arrangement			$ 0	$ 700,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Amount due from agreement			$ 0		$ 0
Collaborative Arrangement | Immuno Gen Inc | Licenses Received
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services		12,700,000
Collaborative Arrangement | Immuno Gen Inc | 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
Collaborative Arrangement | Immuno Gen Inc | 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

Research and Collaboration Ag_9

Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details)	1 Months Ended		3 Months Ended		71 Months Ended
	Dec. 31, 2014USD ($)	May 31, 2013USD ($)Target	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional target as deferred revenue			$ 0
Remaining deferred revenue recognized			29,500,000
Revenue recognized from collaborative arrangement			$ 29,485,000	$ 14,184,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			$ 248,519,000		$ 248,519,000	$ 277,980,000
Collaborative Arrangement | Pfizer Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets | Target		2
Nominated number of additional research targets | Target		2
Additional target as deferred revenue	$ 1,500,000
Remaining deferred revenue recognized				$ 1,100,000
Upfront payment received		$ 6,000,000
Adjusted amortization period of deferred revenue					5 years 6 months
Revenue recognized from collaborative arrangement			$ 0	$ 1,400,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			$ 0	$ 0	$ 0
Amount due from agreement			$ 0	$ 0	$ 0

Research and Collaboration A_10

Research and Collaboration Agreements - Summary of Contract Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Revenue From Contract With Customer [Abstract]
Balance at Beginning of Period	$ 277,980
Deductions	(29,461)
Balance at End of Period	$ 248,519

Research and Collaboration A_11

Research and Collaboration Agreements - Contract Liabilities - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additions to deferred revenue	$ 0
Deductions to deferred revenue recognized during the period	29,500,000
Deferred revenue	248,519,000	$ 277,980,000
AbbVie Ireland Unlimited Company | CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 20,700,000	23,200,000
Deferred revenue recognition maturity date	Apr. 30, 2021
AbbVie Ireland Unlimited Company | Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 4,200,000	$ 4,700,000
Deferred revenue recognition maturity date	Apr. 30, 2021
Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 39,800,000
Deferred revenue recognition maturity date	Sep. 30, 2024
Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 3,600,000
Deferred revenue recognition maturity date	Sep. 30, 2023
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 180,200,000
Deferred revenue recognition maturity date	Apr. 30, 2025

UCSB License Agreement - Additi

UCSB License Agreement - Additional Information (Details) - USD ($)	Apr. 02, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2013
License Agreement [Line Items]
Annual minimum royalty obligations		$ 150,000
License termination period		60 days
Common stock, shares issued		45,157,652	45,083,209
UCSB
License Agreement [Line Items]
Common stock, shares issued				157,332
UCSB | Subsequent Event
License Agreement [Line Items]
Common stock, shares issued	150,000
Annual license maintenance fees	$ 750,000
License payment term	2031

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Activities Under Company's Stock Option Plans (Details)	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Number of Options
Balances, beginning of the period | shares	7,803,773
Options granted | shares	2,103,883
Options exercised | shares	(74,443)
Option forfeited/expired | shares	(61,025)
Balances, end of the period | shares	9,772,188
Options exercisable, end of the period | shares	4,854,438
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period | $ / shares	$ 12.622
Options granted | $ / shares	14.736
Options exercised | $ / shares	6.240
Option forfeited/expired | $ / shares	17.894
Balances, end of the period | $ / shares	13.093
Options exercisable, end of the period | $ / shares	$ 8.558

Stock-Based Compensation - Tota

Stock-Based Compensation - Total Stock-based Compensation Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Stock-based compensation expense:
Stock-based compensation expense	$ 5,192	$ 3,688
Research and development
Stock-based compensation expense:
Stock-based compensation expense	2,642	1,808
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 2,550	$ 1,880

Leases - Additional Information

Leases - Additional Information (Details)	Dec. 10, 2015USD ($)ft²	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)
Operating Leased Assets [Line Items]
Rent expense		$ 1,300,000	$ 1,300,000
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	76,000
Lease term description		The term of the Lease commenced on October 1, 2016. The 2016 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 2016 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

Leases - Summary of Supplementa

Leases - Summary of Supplemental Information Related to Leases (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$ 1,205
Supplemental balance sheet information related to leases:
Operating lease right-of-use assets	27,404
Current operating lease liabilities	2,544
Non-current operating lease liabilities	$ 27,008
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Total operating lease liabilities	$ 29,552
Weighted-average remaining lease term (in years)
Operating lease	7 years 7 months 9 days
Weighted-average discount rate
Operating lease	8.25%
Maturity of operating lease liabilities
2019	$ 4,888
2020	5,025
2021	5,165
2022	5,309
2023 and beyond	19,763
Total lease payments	40,150
Less imputed interest	(10,598)
Present value of lease liabilities	$ 29,552

Income Tax Expense - Additional

Income Tax Expense - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ (6)	$ 1,098
Effective tax rate	0.04%	(7.63%)

Subsequent Event - Additional I

Subsequent Event - Additional Information (Details) - USD ($)	Apr. 02, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2013
Subsequent Event [Line Items]
Common stock, shares issued		45,157,652	45,083,209
UCSB
Subsequent Event [Line Items]
Common stock, shares issued				157,332
UCSB | Subsequent Event
Subsequent Event [Line Items]
Common stock, shares issued	150,000
Payment of upfront fees	$ 1,000,000
Annual license maintenance fees	$ 750,000
License payment term	2031