CytomX Therapeutics (CTMX) 10-Q 5 Nov 20 2020 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Oct. 31, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
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		46,224,402
Entity Current Reporting Status	Yes
Entity File Number	001-37587
Entity Tax Identification Number	27-3521219
Entity Address, Address Line One	151 Oyster Point Blvd
Entity Address, Address Line Two	Suite 400
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	515-3185
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, $0.00001 par value per share
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false
Entity Shell Company	false

CONDENSED BALANCE SHEETS

CONDENSED BALANCE SHEETS - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents		$ 176,810	$ 188,425	[1]
Short-term investments		144,266	107,720	[1]
Accounts receivable		543	13	[1]
Income tax receivable		13,061
Prepaid expenses and other current assets		5,625	7,177	[1]
Total current assets		340,305	303,335	[1]
Property and equipment, net		7,190	7,372	[1]
Intangible assets, net		1,203	1,312	[1]
Goodwill		949	949	[1]
Restricted cash		917	917	[1]
Operating lease right-of-use asset		23,239	25,382	[1]
Other assets		1,379	2,015	[1]
Total assets		375,182	341,282	[1]
Current liabilities:
Accounts payable		3,643	4,158	[1]
Accrued liabilities		23,945	30,051	[1]
Deferred revenue, current portion		74,445	51,381	[1]
Total current liabilities		102,033	85,590	[1]
Deferred revenue, net of current portion		202,560	178,858	[1]
Operating lease liabilities - long term		22,525	24,871	[1]
Other long-term liabilities	[1]		850
Total liabilities		327,118	290,169	[1]
Commitments and contingencies (Note 11)				[1]
Stockholders' equity:
Common stock, $0.00001 par value; 150,000,000 and 75,000,000 shares authorized at September 30, 2020 and December 31, 2019, respectively; 46,223,402 and 45,523,088 shares issued and outstanding at September 30, 2020 and December 31, 2019, respectively		1	1	[1]
Additional paid-in capital		483,524	468,285	[1]
Accumulated other comprehensive (loss) income		(47)	57	[1]
Accumulated deficit		(435,414)	(417,230)	[1]
Total stockholders' equity		48,064	51,113	[1]
Total liabilities and stockholders' equity		375,182	341,282	[1]
Convertible Preferred Stock
Stockholders' equity:
Convertible preferred stock, $0.00001 par value; 10,000,000 shares authorized and no shares issued and outstanding at September 30, 2020 and December 31, 2019.				[1]
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Sep. 30, 2020	Dec. 31, 2019
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	150,000,000	75,000,000
Common stock, shares issued	46,223,402	45,523,088
Common stock, shares outstanding	46,223,402	45,523,088
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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 17,788	$ 10,712	$ 83,989	$ 49,210
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 24,049	$ 27,967	$ 90,929	$ 95,178
General and administrative	8,634	8,463	26,886	27,548
Total operating expenses	32,683	36,430	117,815	122,726
Loss from operations	(14,895)	(25,718)	(33,826)	(73,516)
Interest income	200	1,997	1,730	6,854
Other income (expense), net	(15)	22	1	(126)
Loss before income taxes	(14,710)	(23,699)	(32,095)	(66,788)
Benefit from income taxes	0	0	(13,911)	(6)
Net loss	$ (14,710)	$ (23,699)	$ (18,184)	$ (66,782)
Net loss per share, basic and diluted	$ (0.32)	$ (0.52)	$ (0.40)	$ (1.47)
Shares used to compute net loss per share, basic and diluted	46,195,121	45,418,053	45,992,786	45,294,593
Other comprehensive income (loss):
Unrealized gain (loss) on short-term investments, net of tax	$ (63)	$ (99)	$ (104)	$ 192
Impact of adoption of new accounting pronouncement				11
Comprehensive loss	$ (14,773)	$ (23,798)	$ (18,288)	$ (66,579)

STATEMENTS OF STOCKHOLDERS' EQU

STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total		Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Other Comprehensive Income/(Loss)Cumulative Effect, Period of Adoption, Adjustment	Accumulated Deficit	Accumulated DeficitCumulative Effect, Period of Adoption, Adjustment
Beginning balance at Dec. 31, 2018	$ 130,883		$ 1	$ 445,956	$ (93)	$ 11	$ (314,981)	$ (11)
Beginning balance, shares at Dec. 31, 2018			45,083,209
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201409Member
Exercise of stock options	$ 465			465
Exercise of stock options, shares			74,443
Stock-based compensation	5,192			5,192
Other comprehensive income (loss)	155				155
Net income (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
Beginning balance at Dec. 31, 2018	130,883		$ 1	445,956	(93)	$ 11	(314,981)	$ (11)
Beginning balance, shares at Dec. 31, 2018			45,083,209
Other comprehensive income (loss)	192
Net income (loss)	(66,782)
Ending balance at Sep. 30, 2019	82,109		$ 1	463,773	110		(381,775)
Ending balance, shares at Sep. 30, 2019			45,426,468
Beginning balance at Mar. 31, 2019	122,571		$ 1	451,613	73		(329,116)
Beginning balance, shares at Mar. 31, 2019			45,157,652
Exercise of stock options	33			33
Exercise of stock options, shares			15,124
Issuance of common stock under the ESPP	665			665
Issuance of common stock under the ESPP, shares			81,062
Issuance of common stock pursuant to license agreement	1,602			1,602
Issuance of common stock pursuant to license agreement, shares			150,000
Stock-based compensation	5,454			5,454
Other comprehensive income (loss)	136				136
Net income (loss)	(28,960)						(28,960)
Ending balance at Jun. 30, 2019	101,501		$ 1	459,367	209		(358,076)
Ending balance, shares at Jun. 30, 2019			45,403,838
Exercise of stock options	63			63
Exercise of stock options, shares			22,630
Stock-based compensation	4,343			4,343
Other comprehensive income (loss)	(99)				(99)
Net income (loss)	(23,699)						(23,699)
Ending balance at Sep. 30, 2019	82,109		$ 1	463,773	110		(381,775)
Ending balance, shares at Sep. 30, 2019			45,426,468
Beginning balance at Dec. 31, 2019	51,113	[1]	$ 1	468,285	57		(417,230)
Beginning balance, shares at Dec. 31, 2019			45,523,088
Exercise of stock options	2,157			2,157
Exercise of stock options, shares			395,528
Stock-based compensation	4,013			4,013
Other comprehensive income (loss)	279				279
Net income (loss)	12,205						12,205
Ending balance at Mar. 31, 2020	69,767		$ 1	474,455	336		(405,025)
Ending balance, shares at Mar. 31, 2020			45,918,616
Beginning balance at Dec. 31, 2019	$ 51,113	[1]	$ 1	468,285	57		(417,230)
Beginning balance, shares at Dec. 31, 2019			45,523,088
Exercise of stock options, shares	627,999
Other comprehensive income (loss)	$ (104)
Net income (loss)	(18,184)
Ending balance at Sep. 30, 2020	48,064		$ 1	483,524	(47)		(435,414)
Ending balance, shares at Sep. 30, 2020			46,223,402
Beginning balance at Mar. 31, 2020	69,767		$ 1	474,455	336		(405,025)
Beginning balance, shares at Mar. 31, 2020			45,918,616
Exercise of stock options	1,179			1,179
Exercise of stock options, shares			199,139
Issuance of common stock under the ESPP	369			369
Issuance of common stock under the ESPP, shares			72,315
Stock-based compensation	3,513			3,513
Other comprehensive income (loss)	(320)				(320)
Net income (loss)	(15,679)						(15,679)
Ending balance at Jun. 30, 2020	58,829		$ 1	479,516	16		(420,704)
Ending balance, shares at Jun. 30, 2020			46,190,070
Exercise of stock options	167			167
Exercise of stock options, shares			33,332
Stock-based compensation	3,841			3,841
Other comprehensive income (loss)	(63)				(63)
Net income (loss)	(14,710)						(14,710)
Ending balance at Sep. 30, 2020	$ 48,064		$ 1	$ 483,524	$ (47)		$ (435,414)
Ending balance, shares at Sep. 30, 2020			46,223,402
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

CONDENSED STATEMENTS OF CASH FL

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (18,184)	$ (66,782)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Amortization of intangible assets	109	109
Depreciation and amortization	1,802	1,888
Accretion of discounts on investments	(341)	(1,963)
Stock-based compensation expense	11,367	14,989
Issuance of stock in connection with UCSB sublicense fee		1,602
Changes in operating assets and liabilities
Accounts receivable	(530)	90
Prepaid expenses, income tax receivable and other current assets	(11,509)	193
Other assets	636
Accounts payable	(270)	152
Accrued liabilities, income tax payable and other long-term liabilities	(7,160)	(22,104)
Deferred revenue	46,766	(39,175)
Net cash provided by (used in) operating activities	22,686	(111,001)
Cash flows from investing activities:
Purchases of property and equipment	(1,864)	(2,794)
Purchases of short-term investments	(189,005)	(149,532)
Maturities of short-term investments	152,696	179,170
Net cash (used in) provided by investing activities	(38,173)	26,844
Cash flows from financing activities:
Proceeds from employee stock purchase plan and exercise of stock options	3,872	1,226
Net cash provided by financing activities	3,872	1,226
Net decrease in cash, cash equivalents and restricted cash	(11,615)	(82,931)
Cash, cash equivalents and restricted cash, beginning of period	189,342	248,494
Cash, cash equivalents and restricted cash, end of period	177,727	165,563
Supplemental disclosures of noncash investing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 183	293
Right of use assets obtained in exchange for operating lease obligations		$ 28,054

Description of the Business

Description of the Business	9 Months Ended
	Sep. 30, 2020
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	9 Months Ended
	Sep. 30, 2020
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, 2019 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 and nine months ended September 30, 2020 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, 2019 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. September 30, 2020 December 31, 2019 September 30, 2019 December 31, 2018 (in thousands) Cash and cash equivalents $ 176,810 $ 188,425 $ 164,646 $ 247,577 Restricted cash - non-current assets 917 917 917 917 Total $ 177,727 $ 189,342 $ 165,563 $ 248,494 Short-term Investments All investments have been classified as available-for-sale (“AFS”) and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities less than 12 months are considered short-term investments. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold. The Company assesses impairment of its AFS debt securities investments at each reporting period. Unrealized gains resulting from the excess of the fair value over the amortized cost basis of an investment are reported as a component of accumulated other comprehensive income (loss), net of tax. Unrealized losses or impairments resulting from the fair value of the AFS debt security being below the amortized cost basis are evaluated, using the discounted cash flow model, for identification of credit losses and non-credit related losses. Any credit losses are charged to earnings against the allowance for credit losses of the security, limited to the difference between the fair value and the amortized cost basis of the security. Any difference between the fair value of the security and the amortized cost basis, less the allowance for credit losses, are reported in other comprehensive income (loss). Expected cash inflows due to improvements in credit are recognized through a reversal of the allowance for credit losses subject to the total allowance previously recognized. In the event of impairment of any security, if management (i) has the intent to sell such security or (ii) will more-likely-than-not be required to sell such security before recovery of its amortized cost basis, such AFS debt security’s amortized cost basis will be written down to its fair value through earnings along with any existing allowance for credit losses. Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s non-credit related unrealized gains and losses on short-term investments and impact of adoption of new accounting pronouncements during the period represent the components of other comprehensive income (loss) that are 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 ( “ SSP ” ) of each distinct performance obligation, which requires judgment. In instances where SSP is not directly observable, such as when a license or service is not sold separately, SSP is determined using information that may include market conditions and other observable inputs. Due to the early stage of the Company ’ s licensed technology, the license of such technology is typically combined with research and development services and steering committee participation as one performance obligation. In the event that the Company receives non-cash consideration such as consideration in the form of a research license and research support services from the counterparty, the transaction price of a non-monetary exchange that has commercial substance is estimated based on the fair value of the non-cash consideration received, which may be determined through a valuation analysis. In certain cases, the Company’s performance creates an asset that does not have an alternative use to the customer 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 these 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 from a third party for $5.0 million. The Company plans to use this technology in certain of the Company’s discovery stage projects, and has concluded that the technology acquired does not have an alternative future use. Accordingly, the $5.0 million has been recorded as research and development expense for the nine months ended September 30, 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. Volatility for ESPP shares is equal to the Company’s historical volatility over the six-month offering period. • Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury yield with a maturity equal to the expected term of the stock options in effect at the time of grant. • Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and has no current plan to pay any dividends on its 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 elected the short-term lease recognition exemption . Adopted Accounting Pronouncements Financial Instruments - Credit Losses In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments This new standard also requires that credit losses related to available-for-sale debt securities be recorded as an allowance through net income rather than reducing the carrying amount under the previous other-than-temporary-impairment model. Simplification of the Test for Goodwill Impairment In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment Fair Value Measurement Disclosure Requirements Modification In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) Internal Use Software Guidance for Cloud Computing Arrangement In August 2018, the FASB issued ASU No. 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40) Collaborative Arrangements and Revenue Recognition In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the interaction between Topic 808 and Topic 606 Share-Based Payment to Customer In November 2019, the FASB issued ASU 2019-08, Compensation - Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606): Codification Improvements – Share-Based Consideration Payable to a Customer. The amendments in this ASU require that an entity measure and classify share-based payment awards granted to a customer by applying the guidance in Topic 718. Under ASC 606, these awards are considered a reduction of the transaction price, unless the awards are payment for a distinct good or service received from the customer and should be recorded as a reduction of the transaction price. However, the ASU requires these awards to be measured on the basis of the grant-date fair value of the share-based payment award in accordance with Topic 718 and should be recognized at the later of when the award is promised and when the entity recognizes revenue for the transfer of the related goods or services in accordance with ASC 606. The ASU is effective for the Company on January 1, 2020, and there was no material impact on its financial statements upon adoption of this ASU. Recent Accounting Pronouncements In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	3. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding for the period. Diluted net loss per share is calculated using the weighted-average number of common shares outstanding, plus potential dilutive common stock during the period. Diluted net loss per share was the same as basic net loss per share 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 Nine Months Ended September 30, September 30, 2020 2019 2020 2019 Options and ESPP to purchase common stock 11,561,435 9,841,889 11,414,085 9,562,876

Fair Value Measurements and Sho

Fair Value Measurements and Short-Term Investments	9 Months Ended
	Sep. 30, 2020
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: September 30, 2020 Valuation Hierarchy Amortized Cost Allowance for Credit Losses Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 150,950 $ — $ — $ — $ 150,950 Restricted cash (money market funds) Level I 917 — — — 917 U.S. Government bonds Level I 144,260 — 6 — 144,266 Total $ 296,127 $ — $ 6 $ — $ 296,133 December 31, 2019 Valuation Hierarchy Amortized Cost Allowance for Credit Losses Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 170,757 $ — $ — $ — $ 170,757 Restricted cash (money market funds) Level I 917 — — — 917 U.S. Government bonds Level I 107,610 — 110 — 107,720 Total $ 279,284 $ — $ 110 $ — $ 279,394 As of September 30, 2020, no securities have contractual maturities of greater than 12 months. The unrealized gains on the Company’s investment in US Government bonds were caused by interest rate decreases. The contractual terms of those investments are less than a year and do not permit the issuer to settle the securities at a price less than the amortized cost bases of the investments if held to maturity. The Company does not intend to sell the investments and it is not more likely than not that the Company will be required to sell the investments before recovery of their amortized cost bases .

Accrued Liabilities

Accrued Liabilities	9 Months Ended
	Sep. 30, 2020
Accrued Liabilities Current [Abstract]
Accrued Liabilities	5. Accrued Liabilities Accrued liabilities consisted of the following: September 30, December 31, 2020 2019 (in thousands) Research and clinical expenses $ 12,780 $ 19,006 Payroll and related expenses 6,555 6,721 Legal and professional expenses 1,217 1,062 Operating lease liabilities - short term 3,096 2,810 Other accrued expenses 297 452 Total $ 23,945 $ 30,051

Research and Collaboration Agre

Research and Collaboration Agreements	9 Months Ended
	Sep. 30, 2020
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 Nine Months Ended September 30, September 30, 2020 2019 2020 2019 (in thousands) (in thousands) AbbVie $ 4,209 $ 1,652 $ 35,624 $ 5,983 Amgen 1,633 1,648 7,007 2,905 Astellas 4,543 - 9,133 - Bristol Myers Squibb 7,403 7,412 32,225 40,322 Total revenue $ 17,788 $ 10,712 $ 83,989 $ 49,210 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 percentage if the Company participates in the co-development of the CD71 PDC 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 subsequently earned an additional $10.0 million milestone payment triggered by selection of the second target by AbbVie in June 2019. The Company is also eligible to receive up to $275.0 million in development, regulatory and commercial milestone payments and royalties in the high single to low teens percentage from commercial sales of any resulting PDCs. The second target was selected under the Discovery Agreement that allows AbbVie to select a target for developing a PDC or a Probody. The Company has determined that the AbbVie Agreements 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 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 for the Discovery Agreement and CD71 Agreement, collectively, upon adoption of ASC 606 on January 1, 2018 of $39.8 million consists of $30.0 million in upfront payments, and a $14.0 million milestone payment received under the CD71 Agreement (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 the 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 CX-2029. Therefore, of the $39.8 million total initial transaction price discussed above, the Company allocated $29.8 million to the CD71 Agreement performance obligation and recognized revenue using a cost-based input measure, the common measure of progress for the performance obligation. 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 for completing its combined performance obligation over the estimated service period. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. During 2019, as a result of ongoing dose escalation in the continued development program, there has been a change in estimates of the research service period as well as an increase in the projected FTE hours-to-completion. The research service period for the CD71 Agreement performance obligation was extended from April 2021 to March 2022 in 2019. During the second quarter of 2020, the Company further increased the projected FTE hours-to-completion and extended the research service period for the CD71 Agreement performance obligation from March 2022 to September 2022 in response to the reduced rate of operation as impacted by the COVID-19 pandemic. The remaining $10.0 million of the total initial transaction price of $39.8 million allocated to the Discovery Agreement performance obligation represents an obligation to continuously make the Company ’ five-year 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) under the CD71 Agreement. 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. In June 2019, the Company earned a $10.0 million milestone payment for the second target selected by AbbVie under the Discovery Agreement. It is recognized also using the time-elapse measure of progress of the related obligation and straight line over the estimated research service period of five years through June 2024. The $40.0 million milestone payment earned in March 2020 for satisfying the CD71 dose escalation success criteria under the CD71 Agreement was included as part of the transaction price as it was unconstrained during the first quarter of 2020 and $26.6 million was recognized as revenue related to this milestone, which reflected the percentage completed to-date on the project in March 2020. The remaining $13.4 million will be recognized over the remaining research service period through September 2022. The Company is obligated to make sublicense payments under the license agreement with the Regents of the University of California, acting through its Santa Barbara campus (“UCSB”), as amended, equal to up to 7.5% of certain upfront and milestone payments owed to or received by the Company. As of both September 30, 2020 and December 31, 2019, there was no sublicense fee payable to UCSB. The Company determined that the remaining potential milestone payments of both agreements are probable of significant revenue reversal as their achievement is highly dependent on factors outside the Company ’ The Company recognized revenue of $4.2 million and $1.7 million for the three months ended September 30, 2020 and 2019, respectively, and $35.6 million and $6.0 million for the nine months ended September 30, 2020 and 2019, respectively, related to the AbbVie Agreements. As of September 30, 2020 and December 31, 2019, deferred revenue related to the CD71 Agreement performance obligation was $27.4 million and $20.0 million, respectively, and deferred revenue related to the Discovery Agreement performance obligation was $8.7 million and $11.6 million, respectively. As of both September 30, 2020 and December 31, 2019, no amount was due from AbbVie under the AbbVie Agreements. Amgen, Inc. On September 29, 2017, the Company and Amgen, Inc. ( “ ” “ ” “ ” ’ six-month Under the terms of the Amgen Agreement, the Company and Amgen will co-develop a Probody T-cell engaging bi-specific therapeutic targeting epidermal growth factor receptor (the “ ” “ ” 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. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. 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 both September 30, 2020 and December 31, 2019, the Company recorded no sublicense fee payable to UCSB. 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. At the end of the second quarter of 2019, the Company determined that it will undertake additional testing and assessment of the molecules being evaluated under the EGFR project. As a result, the estimated FTE hours-to-completion and research service period were increased to eight years . In the second quarter of 2020, the Company completed the clinical candidate characterization phase and has mov ed in to the IND-enabling phase earlier than planned . As a result, the estimated FTE hours-to-completion and research service period were decreased from eight to seven years . 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 $1.6 million for each of the three-month periods ended September 30, 2020 and 2019, and $7.0 million and $2.9 million for the nine months ended September 30, 2020 and 2019, respectively, related to the Amgen Agreement. As of September 30, 2020 and December 31, 2019, deferred revenue related to the EGFR Products performance obligation was $31.2 million and $37.6 million, respectively. As of September 30, 2020 and December 31, 2019, deferred revenue related to the Amgen Other Products performance obligation was $2.4 million and $3.0 million, respectively. As of both September 30, 2020 and December 31, 2019, no amount was due from Amgen under the Amgen Agreement. Astellas Pharma Inc. The Company and Astellas Pharma, Inc. (“Astellas”) entered into a Collaboration and License Agreement (the “Astellas Agreement”) on March 23, 2020, the effective date, to collaborate on preclinical research activities to discover and develop certain antibody compounds for the treatment of cancer using the Company’s Probody therapeutic technology. Under the terms of the Astellas Agreement, the Company granted Astellas an exclusive, worldwide, rights to develop and commercialize Probody therapeutics for up to four collaboration targets including one initial target and three additional targets (“Additional Targets”). In addition, Astellas has the right to expand the number of Additional Targets from three up to five (the “Expansion Option”) before the third anniversary of the effective date. Furthermore, for a specified number of targets, at a pre-specified time prior to the initiation of the first pivotal study of a product against such target, the Company may elect to participate in certain development costs and share in the profits generated in the United States with respect to such product (“Cost Share Option”). The Cost Share Option, if exercised, will also provide the option for the Company to co-commercialize such product in the United States. The Company does not consider the Cost Share Option as a performance obligation at the inception of the agreement as the participation is at the Company’s discretion. Pursuant to the Astellas Agreement, the consideration from Astellas is comprised of an upfront fee of $80.0 million and contingent payments for development, regulatory and sales milestones of up to an aggregate of approximately $1.6 billion. If Astellas exercises its Expansion Option for the two Additional Targets, the Company would be eligible to receive additional upfront and milestone payments aggregating to approximately $0.9 billion. The Company identified the following performance obligations at the inception of the Astellas 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 research services and expertise related to the development of the product candidates should be combined with the research services and participation in the joint research committee as one combined performance obligation. The Company concluded, that at the inception of the agreement, Astellas’ Expansion Option for two Additional Targets were not material rights and therefore not considered performance obligations. As such, each option will be accounted for as a separate arrangement upon exercise. The initial transaction price of $90.0 million consists of the upfront fee of $80.0 million and research and development service fees of $10.0 million. The Company determined that the potential development and regulatory milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company ’ The upfront fee of $80.0 million for the combined obligation to continuously make the Probody therapeutic technology platform available to Astellas is recognized on a straight-line basis for the entire performance obligation over the estimated research service period of five years, which ends in March 2025. The research and development service fees, estimated to be $10.0 million, will be recognized when services are provided based on the prescribed FTE rate. 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. The Company recorded a liability upon entering into the Astellas Agreement in the first quarter of 2020 of $6.0 million, representing 7.5% of the $80.0 million upfront payment, as a sublicense fee payable to UCSB, which was fully paid in the second quarter of 2020. The Company recognized revenue of $4.5 million and $9.1 million for the three and nine months ended September 30, 2020, which included the research and development service fee of $0.5 million and $0.7 million for the three and nine months ended September 30, 2020. As of September 30, 2020, deferred revenue relating to the Astellas Agreement was $71.6 million. The amount due from Astellas under the Astellas Agreement was $0.5 million as of September 30, 2020. 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 Bristol Myers Squibb exclusive worldwide rights to develop and commercialize Probody therapeutics for up to four oncology targets. Bristol Myers Squibb 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 Pursuant to the BMS Agreement, the financial consideration from Bristol Myers Squibb was comprised of an upfront payment of $50.0 million, and the Company was initially entitled to receive contingent payments of up to $25.0 million for additional targets and up to an aggregate of $1,192.0 million for development, regulatory and sales milestones. In addition, 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 ’ ’s The Company received an upfront payment of $50.0 million from Bristol Myers Squibb in July 2014. In January and December 2016, Bristol Myers Squibb selected the third and fourth targets, respectively, and paid the Company $10.0 million and $15.0 million, respectively, pursuant to the terms of the BMS Agreement. In December 2016, Bristol Myers Squibb selected a clinical candidate pursuant to the BMS Agreement, which triggered a $2.0 million pre-clinical milestone payment to the Company. In November 2017, the Company recognized a $10.0 million milestone payment from Bristol Myers Squibb upon approval of the investigational new drug application for the CTLA-4-directed Probody therapeutic. On March 17, 2017, the Company and Bristol Myers Squibb entered into Amendment Number 1 to Extend Collaboration and License Agreement (the “BMS ” “ ” Pursuant to the BMS Amendment, the financial consideration from Bristol Myers Squibb is comprised of an upfront payment of $200.0 million and the Company was initially eligible to receive contingent payments for development, regulatory and sales milestones of up to an aggregate of $3,586.0 million for the eight targets. The Company is also entitled to tiered mid-single to low double-digit percentage royalties from potential future sales. The BMS Amendment does not change the term of the Bristol Myers Squibb ’ ’ The initial transaction price for the BMS Agreement and the BMS ’ During the first quarter of 2019, Bristol Myers Squibb terminated pre-clinical activities on three of the first four collaboration targets selected under the original 2014 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 Bristol Myers Squibb for the first target, CTLA-4, as it is still being actively developed by Bristol Myers Squibb. 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 Bristol Myers Squibb 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 continues to be obligated to perform research work under the BMS Amendment executed in March 2017. In February 2020, Bristol Myers Squibb dosed the first patient in the Part 2 cohort expansion portion of its ongoing BMS-986249 clinical study for the CTLA-4 program, which triggered a $10.0 million milestone payment to the Company ’ 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 both September 30, 2020 and December 31, 2019, there was no sublicense fee payable to UCSB. The Company recognized revenue of $7.4 million for each of the three-month periods ended September 30, 2020 and 2019 and $32.2 million and $40.3 million for the nine months ended September 30, 2020 and 2019, respectively. As of September 30, 2020 and December 31, 2019, deferred revenue relating to the BMS Agreement was $135.7 million and $158.0 million, respectively. The amount due from Bristol Myers Squibb under the BMS Agreement was $0 and $13,000 as of September 30, 2020 and December 31, 2019, respectively. ImmunoGen, Inc. In January 2014, the Company and ImmunoGen, Inc. ( “ ” “ ” ’ “ ” ’ ’ ’ “ ” In February 2017, ImmunoGen exercised its first option to obtain a development and commercialization license for one of the two targets. Substitution rights for this first target selection program terminated in February 2017 and ImmunoGen discontinued the program in July 2017. The Company recognized the remaining deferred revenue related to the discontinued program upon the termination of the program. ImmunoGen exercised its second option to obtain a development and commercialization license pursuant to the ImmunoGen Research Agreement (the “ ” 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. In December 2017, the Company entered into the ImmunoGen 2017 License arrangement and extended the Company ’ In February 2020, the Company initiated the first dosing of a patient in the CX-2009 Phase 2 clinical trial and triggered a $3.0 million milestone payment to ImmunoGen pursuant to the CX-2009 License which continued to remain in effect following the termination of the ImmunoGen 2017 License in December 2019. The Company recorded a $3.0 million charge to research and development expense in the first quarter of 2020, in connection with this milestone payment to ImmunoGen. Contract Liabilities The following table presents changes in the Company’s total contract liabilities during the nine months ended September 30, 2020: Balance at Beginning of Period Additions Deductions Balance at End of Period (in thousands) Contract liabilities: Deferred revenue $ 230,239 $ 120,000 $ (73,234 ) $ 277,005 There were $120.0 million additions to deferred revenue during the nine months ended September 30, 2020. Of such amount, $40.0 million was related to the milestone payment triggered by AbbVie’s CD71 dose escalation success criteria which was achieved in March 2020, and an $80.0 million addition was related to the upfront fee payable under the Astellas Agreement entered into in March 2020. Deductions of $73.2 million related to revenue recognized included in the contract liability balance at the beginning of the period plus the revenue recognized related to the $120.0 million additions during the nine months ended September 30, 2020. The Company expects that the $277.0 million of deferred revenue related to the following contracts as of September 30, 2020 will 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 $27.4 million of deferred revenue related to the CD71 Agreement with AbbVie as of September 30, 2020 is expected to be recognized based on actual FTE effort and program progress until approximately September 2022. • The $1.2 million of deferred revenue related to the first target under the Discovery Agreement with AbbVie as of September 30, 2020 is expected to be recognized ratably until approximately April 2021. • The $7.5 million of deferred revenue related to the second target under the Discovery Agreement as of September 30, 2020 is expected to be recognized ratably until approximately June 2024. • The $31.2 million of deferred revenue related to the Amgen EGFR Products as of September 30, 2020 is expected to be recognized based on actual FTE effort and program progress until approximately September 2024. • The $2.4 million of deferred revenue related to the Amgen Other Products as of September 30, 2020 is expected to be recognized ratably until approximately September 2023. • The $71.6 million of deferred revenue related to the Astellas Agreement as of September 30, 2020 is expected to be recognized ratably until approximately March 2025. • The $135.7 million of deferred revenue related to the BMS Agreement as of September 30, 2020 is expected to be recognized ratably until approximately April 2025.

License Agreements

License Agreements	9 Months Ended
	Sep. 30, 2020
Research And Development [Abstract]
License Agreements	7. License Agreements UCSB 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 $0.2 million 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. In April 2019, the Company entered into Amendment No.3 to the UCSB Agreement to adjust and clarify certain sublicense terms (“Amendment No.3”). In connection with the amendment, the Company issued to UCSB 150,000 shares of CytomX common stock with a fair value of $10.68 per share. Under the terms of Amendment No.3, the Company and UCSB agreed to modify the determination of sublicense revenues payable by the Company to UCSB on certain existing collaboration agreements and on collaboration agreements executed subsequent to Amendment No.3. In exchange, the Company agreed to make an upfront payment of $1.0 million as well as additional annual license maintenance fees of $0.8 million through 2031. In the event that the Company terminates the agreement due to material concern of the safety or efficacy of the related technology, 50% of all remaining maintenance fees will become due immediately. Otherwise, all remaining maintenance fees will become due immediately upon early termination of the agreement unless there is a material breach by UCSB. Pursuant to Amendment No.3, the Company recorded in research and development expense a charge of $3.4 million relating to sublicense and maintenance fees representing the 150,000 shares issued with a fair value of $1.6 million, the upfront payment of $1.0 million and the additional annual maintenance fee of $0.8 million during the second quarter of 2019. The Company incurred no sublicence expenses for each of the three-month periods ended September 30, 2020 and 2019, and $9.1 million and $4.2 million for the nine months ended September 30, 2020 and 2019, respectively, under the provisions of the UCSB Agreement. As of September 30, 2020 and December 31, 2019, the sublicense fee payable to UCSB was $0 and $0.2 million, respectively. ImmunoGen In December 2019, the Company entered into the ImmunoGen 2019 License with ImmunoGen to obtain an exclusive license with respect to epithelial cell adhesion molecule (“EpCAM”).

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Common Stock	8. Common Stock In February 2020, the Company entered into an Open Market Sale Agreement (the “Sales Agreement”) with Jefferies LLC (“Jefferies”), to sell shares of the Company’s common stock, par value $0.00001 per share, with aggregate gross sales proceeds of up to $75,000,000, from time to time upon the Company’s request, through an at the market offering under which Jefferies will act as sales agent. Pursuant to the Sales Agreement, Jefferies as the sales agent will receive a commission of 3.0% of the gross sales price for shares of common stock sold under the Sales Agreement. There were no shares sold during the three and nine months ended September 30, 2020. In June 2020, the Board of Directors and stockholders of the Company approved an increase in the authorized shares of common stock from 75,000,000 to 150,000,000.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation Stock Options Activities under the Company’s stock option plans for the nine months ended September 30, 2020 were as follows: Options Outstanding Number of Options Weighted- Average Exercise Price Per Share Balances at December 31, 2019 9,936,168 $ 12.26 Options granted 3,762,244 7.23 Options exercised (627,999 ) 5.58 Option forfeited/expired (1,688,953 ) 13.94 Balances at September 30, 2020 11,381,460 $ 10.72 Options exercisable at September 30, 2020 6,214,494 $ 11.15 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 Nine Months Ended September 30, September 30, 2020 2019 2020 2019 (in thousands) Stock-based compensation expense: Research and development $ 1,735 $ 1,948 $ 5,317 $ 7,241 General and administrative 2,106 2,395 6,050 7,748 Total stock-based compensation expense $ 3,841 $ 4,343 $ 11,367 $ 14,989

Leases

Leases	9 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Leases	10. 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 September 30, 2020 and December 31, 2019, 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 September 30, 2020 and 2019 was $1.3 million. Rent expense for both the nine months ended September 30, 2020 and 2019 was $3.8 Supplemental information related to leases are as follows: Three Months Ended Nine Months Ended September 30, 2020 September 30, 2019 September 30, 2020 September 30, 2019 (in thousands) (in thousands) Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 1,239 $ 1,205 $ 3,717 $ 3,615 September 30, 2020 December 31, 2019 (in thousands) Supplemental balance sheet information related to leases: Operating lease right-of-use assets $ 23,239 $ 25,382 Current operating lease liabilities 3,096 2,810 Non-current operating lease liabilities 22,525 24,871 Total operating lease liabilities $ 25,621 $ 27,681 Weighted-average remaining lease term (in years) Operating lease 6.00 6.85 Weighted-average discount rate Operating lease 8.25 % 8.25 % September 30, 2020 (in thousands) Maturity of operating lease liabilities 2020 $ 1,274 2021 5,129 2022 5,273 2023 5,420 2024 and beyond 15,689 Total lease payments 32,785 Less imputed interest (7,164 ) Present value of lease liabilities $ 25,621

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Legal Proceedings On March 4, 2020, Vytacera Bio, LLC filed a patent infringement lawsuit against the Company in the U.S. District Court for the District of Delaware. The lawsuit alleges that the Company’s use, offers to sell, and/or sales of the Probody® technology platform for basic research applications constitutes infringement. The complaint seeks unspecified monetary damages. The Company filed an Answer, Affirmative Defenses, and Counterclaims on May 26, 2020. Vytacera Bio, LLC filed its Answer to CytomX Therapeutics Inc.’s Counterclaims on June 5, 2020. The parties have agreed to a case schedule, which is pending Court approval. Discovery is in the initial phases. The Company believes that the lawsuit is without merit and intends to vigorously defend itself and has not recorded any amount for claims associated with this lawsuit as of September 30, 2020. On May 21, 2020, a putative securities class action lawsuit was commenced in the U.S. District Court for the Northern District of California naming as defendants the Company and three current and former officers. The complaint alleges that the defendants violated Sections 10(b) and 20(a) of the Securities Exchange Act of 1934, as amended, and Rule 10b-5 thereunder, by making false and misleading statements and omissions of material fact related to the product candidates CX-072 and CX-2009. The plaintiff seeks to represent all persons who purchased or otherwise acquired CytomX securities between May 17, 2018, and May 13, 2020. The plaintiff seeks damages and interest, and an award of costs, including attorneys’ fees. The Company believes the plaintiff’s claims are without merit and has not recorded any amount for claims associated with this lawsuit as of September 30, 2020.

Income Tax Expense

Income Tax Expense	9 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Tax Expense	12. Income Tax Expense The Company recorded no income tax expense for the three months ended September 30, 2020 and 2019. The Company recorded an income tax benefit of $13.9 million and $6,000 for the nine months ended September 30, 2020 and 2019, respectively. The Company records the effect of an enacted change in a tax law in the period that includes the enactment date in accordance with ASC 740. On March 27, 2020, the Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) was enacted in response to the COVID-19 pandemic. The tax relief measures under the CARES Act for businesses include a five-year The income tax benefit for the nine months ended September 30, 2020 was generated as a result of the recognition of net operating loss carryback under the CARES Act which generated a refund of income taxes paid for 2018. The income tax benefit for the nine months ended September 30, 2019 was as a result of an unrealized gain on the available for sale securities recorded in other comprehensive income during the period. The Company maintains a full valuation allowance against its net deferred tax assets due to the Company’s history of losses as of September 30, 2020 .

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2020
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, 2019 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 and nine months ended September 30, 2020 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, 2019 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. September 30, 2020 December 31, 2019 September 30, 2019 December 31, 2018 (in thousands) Cash and cash equivalents $ 176,810 $ 188,425 $ 164,646 $ 247,577 Restricted cash - non-current assets 917 917 917 917 Total $ 177,727 $ 189,342 $ 165,563 $ 248,494
Short-term Investments	Short-term Investments All investments have been classified as available-for-sale (“AFS”) and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities less than 12 months are considered short-term investments. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold. The Company assesses impairment of its AFS debt securities investments at each reporting period. Unrealized gains resulting from the excess of the fair value over the amortized cost basis of an investment are reported as a component of accumulated other comprehensive income (loss), net of tax. Unrealized losses or impairments resulting from the fair value of the AFS debt security being below the amortized cost basis are evaluated, using the discounted cash flow model, for identification of credit losses and non-credit related losses. Any credit losses are charged to earnings against the allowance for credit losses of the security, limited to the difference between the fair value and the amortized cost basis of the security. Any difference between the fair value of the security and the amortized cost basis, less the allowance for credit losses, are reported in other comprehensive income (loss). Expected cash inflows due to improvements in credit are recognized through a reversal of the allowance for credit losses subject to the total allowance previously recognized. In the event of impairment of any security, if management (i) has the intent to sell such security or (ii) will more-likely-than-not be required to sell such security before recovery of its amortized cost basis, such AFS debt security’s amortized cost basis will be written down to its fair value through earnings along with any existing allowance for credit losses.
Comprehensive Loss	Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s non-credit related unrealized gains and losses on short-term investments and impact of adoption of new accounting pronouncements during the period represent the components of other comprehensive income (loss) that are 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 ( “ SSP ” ) of each distinct performance obligation, which requires judgment. In instances where SSP is not directly observable, such as when a license or service is not sold separately, SSP is determined using information that may include market conditions and other observable inputs. Due to the early stage of the Company ’ s licensed technology, the license of such technology is typically combined with research and development services and steering committee participation as one performance obligation. In the event that the Company receives non-cash consideration such as consideration in the form of a research license and research support services from the counterparty, the transaction price of a non-monetary exchange that has commercial substance is estimated based on the fair value of the non-cash consideration received, which may be determined through a valuation analysis. In certain cases, the Company’s performance creates an asset that does not have an alternative use to the customer 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 these 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 from a third party for $5.0 million. The Company plans to use this technology in certain of the Company’s discovery stage projects, and has concluded that the technology acquired does not have an alternative future use. Accordingly, the $5.0 million has been recorded as research and development expense for the nine months ended September 30, 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. Volatility for ESPP shares is equal to the Company’s historical volatility over the six-month offering period. • Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury yield with a maturity equal to the expected term of the stock options in effect at the time of grant. • Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and has no current plan to pay any dividends on its 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 elected the short-term lease recognition exemption .
Adopted Accounting Pronouncements and Recent Accounting Pronouncements	Adopted Accounting Pronouncements Financial Instruments - Credit Losses In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments This new standard also requires that credit losses related to available-for-sale debt securities be recorded as an allowance through net income rather than reducing the carrying amount under the previous other-than-temporary-impairment model. Simplification of the Test for Goodwill Impairment In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment Fair Value Measurement Disclosure Requirements Modification In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) Internal Use Software Guidance for Cloud Computing Arrangement In August 2018, the FASB issued ASU No. 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40) Collaborative Arrangements and Revenue Recognition In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the interaction between Topic 808 and Topic 606 Share-Based Payment to Customer In November 2019, the FASB issued ASU 2019-08, Compensation - Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606): Codification Improvements – Share-Based Consideration Payable to a Customer. The amendments in this ASU require that an entity measure and classify share-based payment awards granted to a customer by applying the guidance in Topic 718. Under ASC 606, these awards are considered a reduction of the transaction price, unless the awards are payment for a distinct good or service received from the customer and should be recorded as a reduction of the transaction price. However, the ASU requires these awards to be measured on the basis of the grant-date fair value of the share-based payment award in accordance with Topic 718 and should be recognized at the later of when the award is promised and when the entity recognizes revenue for the transfer of the related goods or services in accordance with ASC 606. The ASU is effective for the Company on January 1, 2020, and there was no material impact on its financial statements upon adoption of this ASU. Recent Accounting Pronouncements In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2020
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. September 30, 2020 December 31, 2019 September 30, 2019 December 31, 2018 (in thousands) Cash and cash equivalents $ 176,810 $ 188,425 $ 164,646 $ 247,577 Restricted cash - non-current assets 917 917 917 917 Total $ 177,727 $ 189,342 $ 165,563 $ 248,494

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2020
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 Nine Months Ended September 30, September 30, 2020 2019 2020 2019 Options and ESPP to purchase common stock 11,561,435 9,841,889 11,414,085 9,562,876

Fair Value Measurements and S_2

Fair Value Measurements and Short-Term Investments (Tables)	9 Months Ended
	Sep. 30, 2020
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: September 30, 2020 Valuation Hierarchy Amortized Cost Allowance for Credit Losses Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 150,950 $ — $ — $ — $ 150,950 Restricted cash (money market funds) Level I 917 — — — 917 U.S. Government bonds Level I 144,260 — 6 — 144,266 Total $ 296,127 $ — $ 6 $ — $ 296,133 December 31, 2019 Valuation Hierarchy Amortized Cost Allowance for Credit Losses Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 170,757 $ — $ — $ — $ 170,757 Restricted cash (money market funds) Level I 917 — — — 917 U.S. Government bonds Level I 107,610 — 110 — 107,720 Total $ 279,284 $ — $ 110 $ — $ 279,394

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	9 Months Ended
	Sep. 30, 2020
Accrued Liabilities Current [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consisted of the following: September 30, December 31, 2020 2019 (in thousands) Research and clinical expenses $ 12,780 $ 19,006 Payroll and related expenses 6,555 6,721 Legal and professional expenses 1,217 1,062 Operating lease liabilities - short term 3,096 2,810 Other accrued expenses 297 452 Total $ 23,945 $ 30,051

Research and Collaboration Ag_2

Research and Collaboration Agreements (Tables)	9 Months Ended
	Sep. 30, 2020
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 Nine Months Ended September 30, September 30, 2020 2019 2020 2019 (in thousands) (in thousands) AbbVie $ 4,209 $ 1,652 $ 35,624 $ 5,983 Amgen 1,633 1,648 7,007 2,905 Astellas 4,543 - 9,133 - Bristol Myers Squibb 7,403 7,412 32,225 40,322 Total revenue $ 17,788 $ 10,712 $ 83,989 $ 49,210
Summary of Contract Liabilities	The following table presents changes in the Company’s total contract liabilities during the nine months ended September 30, 2020: Balance at Beginning of Period Additions Deductions Balance at End of Period (in thousands) Contract liabilities: Deferred revenue $ 230,239 $ 120,000 $ (73,234 ) $ 277,005

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2020
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 nine months ended September 30, 2020 were as follows: Options Outstanding Number of Options Weighted- Average Exercise Price Per Share Balances at December 31, 2019 9,936,168 $ 12.26 Options granted 3,762,244 7.23 Options exercised (627,999 ) 5.58 Option forfeited/expired (1,688,953 ) 13.94 Balances at September 30, 2020 11,381,460 $ 10.72 Options exercisable at September 30, 2020 6,214,494 $ 11.15
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 Nine Months Ended September 30, September 30, 2020 2019 2020 2019 (in thousands) Stock-based compensation expense: Research and development $ 1,735 $ 1,948 $ 5,317 $ 7,241 General and administrative 2,106 2,395 6,050 7,748 Total stock-based compensation expense $ 3,841 $ 4,343 $ 11,367 $ 14,989

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Summary of Supplemental Information Related to Leases	Supplemental information related to leases are as follows: Three Months Ended Nine Months Ended September 30, 2020 September 30, 2019 September 30, 2020 September 30, 2019 (in thousands) (in thousands) Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 1,239 $ 1,205 $ 3,717 $ 3,615 September 30, 2020 December 31, 2019 (in thousands) Supplemental balance sheet information related to leases: Operating lease right-of-use assets $ 23,239 $ 25,382 Current operating lease liabilities 3,096 2,810 Non-current operating lease liabilities 22,525 24,871 Total operating lease liabilities $ 25,621 $ 27,681 Weighted-average remaining lease term (in years) Operating lease 6.00 6.85 Weighted-average discount rate Operating lease 8.25 % 8.25 %
Schedule of Maturity of Operating Lease Liabilities	September 30, 2020 (in thousands) Maturity of operating lease liabilities 2020 $ 1,274 2021 5,129 2022 5,273 2023 5,420 2024 and beyond 15,689 Total lease payments 32,785 Less imputed interest (7,164 ) Present value of lease liabilities $ 25,621

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	Sep. 30, 2020	Dec. 31, 2019		Sep. 30, 2019	Dec. 31, 2018
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 176,810	$ 188,425	[1]	$ 164,646	$ 247,577
Restricted cash - non-current assets	917	917	[1]	917	917
Total	$ 177,727	$ 189,342		$ 165,563	$ 248,494
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

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		9 Months Ended
	Jan. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Summary Of Significant Accounting Policies [Line Items]
Research and development expense		$ 24,049	$ 27,967	$ 90,929	$ 95,178
ASU 2016-13
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2020		Jan. 1, 2020
Change in accounting principle, accounting standards update, adopted [true false]		true		true
ASU 2017-04
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2020		Jan. 1, 2020
Change in accounting principle, accounting standards update, adopted [true false]		true		true
Change in accounting principle, accounting standards update, immaterial effect [true false]		true		true
ASU 2018-13
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2020		Jan. 1, 2020
Change in accounting principle, accounting standards update, adopted [true false]		true		true
Change in accounting principle, accounting standards update, immaterial effect [true false]		true		true
ASU 2018-15
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2020		Jan. 1, 2020
Change in accounting principle, accounting standards update, adopted [true false]		true		true
Change in accounting principle, accounting standards update, immaterial effect [true false]		true		true
ASU 2018-18
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2020		Jan. 1, 2020
Change in accounting principle, accounting standards update, adopted [true false]		true		true
Change in accounting principle, accounting standards update, immaterial effect [true false]		true		true
ASU 2019-08
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2020		Jan. 1, 2020
Change in accounting principle, accounting standards update, adopted [true false]		true		true
Change in accounting principle, accounting standards update, immaterial effect [true false]		true		true
ASU 2019-12
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2021		Jan. 1, 2021
Change in accounting principle, accounting standards update, adopted [true false]		false		false
Change in accounting principle, accounting standards update, immaterial effect [true false]		false		false
Probody technology
Summary Of Significant Accounting Policies [Line Items]
Acquired, technology know-how	$ 5,000
Research and development expense				$ 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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Options and ESPP 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	11,561,435	9,841,889	11,414,085	9,562,876

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	Sep. 30, 2020	Dec. 31, 2019		Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 296,127	$ 279,284
Gross Unrealized Holding Gains	6	110
Aggregate Fair Value	296,133	279,394
Cash and cash equivalents	176,810	188,425	[1]	$ 164,646	$ 247,577
Level I | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	150,950	170,757
Level I | Restricted cash (money market funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted cash	917	917
Level I | U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term Investments, Amortized Cost	144,260	107,610
Short-term Investments, Gross Unrealized Holding Gains	6	110
Short-term Investments, Aggregate Fair Value	$ 144,266	$ 107,720
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Fair Value Measurements and S_4

Fair Value Measurements and Short-Term Investments - Additional Information (Details)	Sep. 30, 2020USD ($)
Fair Value Disclosures [Abstract]
Contractual maturities of greater than 12 months	$ 0

Accrued Liabilities - Summary o

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Research and clinical expenses	$ 12,780	$ 19,006
Payroll and related expenses	6,555	6,721
Legal and professional expenses	1,217	1,062
Operating lease liabilities - short term	3,096	2,810
Other accrued expenses	297	452
Total	$ 23,945	$ 30,051	[1]
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Research and Collaboration Ag_3

Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partners (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 17,788	$ 10,712	$ 83,989	$ 49,210
AbbVie
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	4,209	1,652	35,624	5,983
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	1,633	1,648	7,007	2,905
Astellas
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	4,543		9,133
Bristol Myers Squibb
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 7,403	$ 7,412	$ 32,225	$ 40,322

Research and Collaboration Ag_4

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)

1 Months Ended

3 Months Ended

9 Months Ended

Mar. 31, 2020USD ($)

Jun. 30, 2019USD ($)

May 31, 2018USD ($)

Jul. 31, 2017USD ($)

Apr. 30, 2016USD ($)AgreementTargetAccountingUnit

Sep. 30, 2020USD ($)

Sep. 30, 2019USD ($)

Jun. 30, 2018USD ($)

Sep. 30, 2020USD ($)

Sep. 30, 2019USD ($)

Dec. 31, 2019USD ($)

Mar. 31, 2017Target

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized from collaborative arrangement

$ 17,788,000

$ 10,712,000

$ 83,989,000

$ 49,210,000

Type of Revenue [Extensible List]

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

Accrued liabilities

$ 23,945,000

$ 23,945,000

$ 30,051,000

[1]

Deferred revenue

277,005,000

277,005,000

230,239,000

AbbVie Ireland Unlimited Company

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payment received

$ 21,000,000

Revenue recognized from collaborative arrangement

$ 4,200,000

$ 1,700,000

$ 9,900,000

$ 35,600,000

$ 6,000,000

Type of Revenue [Extensible List]

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

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

0

AbbVie Ireland Unlimited Company | CD71 Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment received

$ 20,000,000

Milestone payment received

$ 40,000,000

21,000,000

$ 14,000,000

Milestone payment received

21,000,000

Revenue recognition upon performance obligation

13,400,000

13,400,000

Deferred revenue

27,400,000

27,400,000

20,000,000

AbbVie Ireland Unlimited Company | CD71 Agreement | Transferred over Time

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized from collaborative arrangement

$ 26,600,000

Type of Revenue [Extensible List]

us-gaap:LicenseAndServiceMember

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

$ 1,000,000

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

Deferred revenue

8,700,000

8,700,000

11,600,000

AbbVie Ireland Unlimited Company | Discovery Agreement | ASC 606

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment received

10,000,000

Total transaction price

$ 10,000,000

AbbVie Ireland Unlimited Company | Discovery Agreement | Second Target

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payments received

$ 10,000,000

AbbVie Ireland Unlimited Company | Discovery Agreement | Second Target | ASC 606

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Total transaction price

$ 10,000,000

Estimated research service period

5 years

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 | License Agreement | UC Regents

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

License payment percentage

7.50%

Accrued liabilities

$ 0

$ 0

$ 0

[1]

The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Research and Collaboration Ag_5

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details 1)	Apr. 30, 2016
AbbVie Ireland Unlimited Company | Discovery Agreement | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2016-04-01
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)	Sep. 29, 2017USD ($)Target	Oct. 31, 2017USD ($)$ / sharesshares	Sep. 30, 2020USD ($)	Jun. 30, 2020	Sep. 30, 2019USD ($)	Jun. 30, 2019	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities			$ 23,945,000				$ 23,945,000		$ 30,051,000	[1]
Revenue recognized from collaborative arrangement			$ 17,788,000		$ 10,712,000		$ 83,989,000	$ 49,210,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			$ 277,005,000				$ 277,005,000		230,239,000
Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			31,200,000				31,200,000
Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			2,400,000				2,400,000
Collaboration and License Agreement | Amgen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 40,000,000	$ 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			$ 1,600,000		$ 1,600,000		$ 7,000,000	$ 2,900,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Amount due from agreement			$ 0				$ 0		0
Collaboration and License Agreement | Amgen Inc | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments payable	203,000,000
Collaboration and License Agreement | Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price	46,400,000
Estimated research service period				7 years		8 years
Deferred revenue			31,200,000				31,200,000		37,600,000
Collaboration and License Agreement | Amgen Inc | EGFR Products | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 455,000,000
Percentage share of profit and losses	50.00%
Collaboration and License Agreement | Amgen Inc | Amgen Products | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 950,000,000
Number of targets | Target	3
Collaboration and License Agreement | Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price	$ 4,800,000
Estimated research service period	6 years
Revenue recognition upon performance obligation	$ 4,800,000
Deferred revenue			2,400,000				2,400,000		3,000,000
Sublicense Agreement | Amgen Inc | UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities			$ 0				$ 0		$ 0
Sublicense Agreement | Amgen Inc | Maximum | UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License payment percentage	7.50%
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Research and Collaboration Ag_7

Research and Collaboration Agreements - Astellas Pharma Inc - Additional Information (Details)	Mar. 23, 2020USD ($)Target	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities		$ 23,945,000		$ 23,945,000		$ 30,051,000	[1]
Revenue recognized from collaborative arrangement		17,788,000	$ 10,712,000	83,989,000	$ 49,210,000
Deferred revenue		277,005,000		277,005,000		230,239,000
UCSB
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities		0		0		$ 200,000
Astellas Pharma Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue		71,600,000		71,600,000
Collaboration and License Agreement | Astellas Pharma Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets selected | Target	1
Number of additional collaboration target | Target	3
Upfront payment received	$ 80,000,000			80,000,000
Total transaction price	90,000,000
Research and development service fees	$ 10,000,000	500,000		700,000
Estimated research service period	5 years
Estimated research service termination month & year	2025-03
Revenue recognized from collaborative arrangement		4,500,000		9,100,000
Deferred revenue		71,600,000		71,600,000
Amount due from agreement		500,000		500,000
Collaboration and License Agreement | Astellas Pharma Inc. | UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License payment percentage	7.50%
Collaboration and License Agreement | Astellas Pharma Inc. | UCSB
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities		$ 6,000,000		$ 6,000,000
Collaboration and License Agreement | Astellas Pharma Inc. | Additional Contingent Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 900,000,000
Collaboration and License Agreement | Astellas Pharma Inc. | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets | Target	4
Right to expand the number of additional collaboration target | Target	5
Contingent milestone payments receivable	$ 1,600,000,000
Collaboration and License Agreement | Astellas Pharma Inc. | Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Right to expand the number of additional collaboration target | Target	3
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Research and Collaboration Ag_8

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)

Apr. 25, 2017USD ($)Target

Jul. 07, 2014USD ($)TargetTerm

Sep. 30, 2020USD ($)

Mar. 31, 2020USD ($)

Sep. 30, 2019USD ($)

Mar. 31, 2019USD ($)

Sep. 30, 2020USD ($)

Sep. 30, 2019USD ($)

Feb. 29, 2020USD ($)

Dec. 31, 2019USD ($)

Nov. 30, 2017USD ($)

Dec. 31, 2016USD ($)

Jan. 31, 2016USD ($)

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized from collaborative arrangement

$ 17,788,000

$ 10,712,000

$ 83,989,000

$ 49,210,000

Accrued liabilities

$ 23,945,000

$ 23,945,000

$ 30,051,000

[1]

Type of Revenue [Extensible List]

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

Deferred revenue

$ 277,005,000

$ 277,005,000

230,239,000

UCSB

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Accrued liabilities

0

0

200,000

Bristol Myers Squibb Company

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Deferred revenue

135,700,000

$ 135,700,000

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 Bristol Myers Squibb 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

Contingent milestone payments receivable

$ 10,000,000

$ 15,000,000

$ 10,000,000

Number of research targets selected | Target

8

Total transaction price

272,800,000

Upfront payment received

250,000,000

Research and development service fees

10,800,000

Milestone payment received

$ 12,000,000

$ 10,000,000

Estimated research service termination date

Apr. 25, 2025

Revenue recognized from collaborative arrangement

$ 7,400,000

$ 7,400,000

$ 32,200,000

$ 40,300,000

Type of Revenue [Extensible List]

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

Deferred revenue

$ 135,700,000

$ 135,700,000

158,000,000

Amount due from agreement

0

0

13,000

Collaborative Arrangement | Bristol Myers Squibb Company | UC Regents

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

License payment percentage

7.50%

Collaborative Arrangement | Bristol Myers Squibb Company | UCSB

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Accrued liabilities

$ 0

$ 0

$ 0

Collaborative Arrangement | Bristol Myers Squibb Company | Pre-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

Contingent milestone payments receivable

$ 3,586,000,000

$ 1,192,000,000

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Each Of Two Additional Targets

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 25,000,000

Collaboration and License Agreement | Maximum | Bristol Myers Squibb Company

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Number of additional collaboration target | Target

8

[1]

The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Research and Collaboration Ag_9

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details)	1 Months Ended	3 Months Ended			9 Months Ended
	Jan. 31, 2014USD ($)Target	Sep. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Feb. 29, 2020USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue		$ 277,005,000			$ 277,005,000			$ 230,239,000
Charge to research and development expense		$ 24,049,000		$ 27,967,000	$ 90,929,000	$ 95,178,000
Collaborative Arrangement | Immuno Gen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	$ 0
Number of research targets selected | Target	2
Estimated total fair value consideration recorded as deferred revenue	$ 13,200,000
Milestone payments payable							$ 3,000,000
Charge to research and development expense			$ 3,000,000

Research and Collaboration A_10

Research and Collaboration Agreements - Summary of Contract Liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020USD ($)
Revenue From Contract With Customer [Abstract]
Balance at Beginning of Period	$ 230,239
Additions	120,000
Deductions	(73,234)
Balance at End of Period	$ 277,005

Research and Collaboration A_11

Research and Collaboration Agreements - Contract Liabilities - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additions to deferred revenue	$ 120,000
Deductions to deferred revenue recognized during the period	73,234
Deferred revenue	277,005	$ 230,239
AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additions to deferred revenue	40,000
AbbVie Ireland Unlimited Company | CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 27,400	$ 20,000
Deferred revenue recognition maturity month and year	2022-09
AbbVie Ireland Unlimited Company | First Target under Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 1,200
Deferred revenue recognition maturity month and year	2021-04
AbbVie Ireland Unlimited Company | Second Target Under Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 7,500
Deferred revenue recognition maturity month and year	2024-06
Astellas Pharma Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additions to deferred revenue	$ 80,000
Deferred revenue	$ 71,600
Deferred revenue recognition maturity month and year	2025-03
Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 31,200
Deferred revenue recognition maturity month and year	2024-09
Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 2,400
Deferred revenue recognition maturity month and year	2023-09
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 135,700
Deferred revenue recognition maturity month and year	2025-04

License Agreements - Additional

License Agreements - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended				9 Months Ended
	Apr. 30, 2019	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019		Dec. 31, 2013
License Agreement [Line Items]
Annual minimum royalty obligations		$ 200				$ 200
License termination period						60 days
Common stock, shares issued		46,223,402				46,223,402		45,523,088
Research and development		$ 24,049		$ 27,967		$ 90,929	$ 95,178
Accrued liabilities		23,945				23,945		$ 30,051	[1]
Immuno Gen Inc | Collaborative Arrangement
License Agreement [Line Items]
Research and development			$ 3,000
Upfront license payment								7,500
UCSB
License Agreement [Line Items]
Common stock, shares issued	150,000									157,332
Fair value of common stock price per share	$ 10.68
Payment of upfront fees	$ 1,000				$ 1,000
Annual license maintenance fees	$ 800				$ 800
License payment term	2031
Percentage of remaining maintenance fees outstanding	50.00%
Fair value of common stock issued	$ 1,600
Sublicense fees		0		$ 0		9,100	$ 4,200
Accrued liabilities		$ 0				$ 0		$ 200
UCSB | Sublicense and Maintenance Fees
License Agreement [Line Items]
Research and development	$ 3,400
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Common Stock - Additional Infor

Common Stock - Additional Information (Details) - USD ($)	1 Months Ended
	Feb. 29, 2020	Sep. 30, 2020	Jun. 30, 2020	May 31, 2020	Dec. 31, 2019
Class Of Stock [Line Items]
Common stock, par value		$ 0.00001			$ 0.00001
Common stock, shares issued		46,223,402			45,523,088
Common stock, shares authorized		150,000,000	150,000,000	75,000,000	75,000,000
with Jefferies LLC
Class Of Stock [Line Items]
Common stock, par value	$ 0.00001
Percentage of sales commission	3.00%
Common stock, shares issued		0
with Jefferies LLC | Maximum
Class Of Stock [Line Items]
Common stock, value of shares issued in connection with agreement	$ 75,000,000

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Activities Under Company's Stock Option Plans (Details) - $ / shares	9 Months Ended
	Sep. 30, 2020
Number of Options
Balances, beginning of the period	9,936,168
Options granted	3,762,244
Options exercised	(627,999)
Option forfeited/expired	(1,688,953)
Balances, end of the period	11,381,460
Options exercisable, end of the period	6,214,494
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period	$ 12.26
Options granted	7.23
Options exercised	5.58
Option forfeited/expired	13.94
Balances, end of the period	10.72
Options exercisable, end of the period	$ 11.15

Stock-Based Compensation - Tota

Stock-Based Compensation - Total Stock-based Compensation Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Stock-based compensation expense:
Stock-based compensation expense	$ 3,841	$ 4,343	$ 11,367	$ 14,989
Research and development
Stock-based compensation expense:
Stock-based compensation expense	1,735	1,948	5,317	7,241
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 2,106	$ 2,395	$ 6,050	$ 7,748

Leases - Additional Information

Leases - Additional Information (Details)	Dec. 10, 2015USD ($)ft²	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)		Dec. 31, 2018USD ($)
Operating Leased Assets [Line Items]
Restricted cash		$ 917,000	$ 917,000	$ 917,000	$ 917,000	$ 917,000	[1]	$ 917,000
Rent expense		1,300,000	$ 1,300,000	3,800,000	$ 3,800,000
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit outstanding, amount		900,000		900,000
Restricted cash		$ 900,000		$ 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
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Leases - Summary of Supplementa

Leases - Summary of Supplemental Information Related to Leases (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$ 1,239	$ 1,205	$ 3,717	$ 3,615
Supplemental balance sheet information related to leases:
Operating lease right-of-use assets	23,239		23,239		$ 25,382	[1]
Current operating lease liabilities	3,096		3,096		2,810
Non-current operating lease liabilities	$ 22,525		$ 22,525		$ 24,871	[1]
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent		us-gaap:OtherLiabilitiesCurrent		us-gaap:OtherLiabilitiesCurrent
Total operating lease liabilities	$ 25,621		$ 25,621		$ 27,681
Weighted-average remaining lease term (in years)
Operating lease	6 years		6 years		6 years 10 months 6 days
Weighted-average discount rate
Operating lease	8.25%		8.25%		8.25%
[1]	The condensed balance sheet as of December 31, 2019 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Leases - Schedule of Maturity o

Leases - Schedule of Maturity of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Maturity of operating lease liabilities
2020	$ 1,274
2021	5,129
2022	5,273
2023	5,420
2024 and beyond	15,689
Total lease payments	32,785
Less imputed interest	(7,164)
Present value of lease liabilities	$ 25,621	$ 27,681

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - Patent Infringement Lawsuit	May 21, 2020USD ($)Defendant	Mar. 04, 2020USD ($)	Sep. 30, 2020
Loss Contingencies [Line Items]
Lawsuit filed date		March 4, 2020
Name of plaintiff			Vytacera Bio, LLC
Loss contingency, answer, affirmative defenses and counterclaims filing date		May 26, 2020
Loss contingency, answer and counterclaims by plaintiff filing date		Jun. 5, 2020
Loss contingency claim amount | $	$ 0	$ 0
Number of current and former officers named as defendants | Defendant	3

Income Tax Expense - Additional

Income Tax Expense - Additional Information (Details) - USD ($) $ in Thousands	Mar. 27, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income tax (benefit) expense		$ 0	$ 0	$ (13,911)	$ (6)
Net operating loss carryback	5 years
Percentage of taxable income from net operating losses for suspension of annual deduction limitation	80.00%