Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash Flows Used in Operating Activities
Net loss	$ (115,088)	$ (90,137)	$ (89,860)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	280	364	479
Accretion (amortization) of premiums and discounts on investments	2	(142)	(897)
Stock-based compensation	10,264	8,060	6,466
Gain on remeasurement of the lease liability	(362)
Non-cash lease expense	1,134	1,506	1,366
Series A convertible preferred stock issued for research and development related to asset acquisition	40,875
Changes in operating assets and liabilities:
Receivables from collaborations	1,541	(1,059)	1,515
Prepaid expenses and other current assets	96	(58)	(26)
Other assets		280	289
Accounts payable	1,656	(58)	816
Accrued liabilities	(5,523)	(1,590)	2,576
Lease liability	(1,175)	(1,478)	(1,469)
Net cash used in operating activities	(66,300)	(84,312)	(78,745)
Cash Flows Provided by (Used in) Investing Activities
Purchases of investments		(57,059)	(153,227)
Proceeds from the sale and maturity of investments	8,000	146,080	142,450
Purchase of property and equipment	(147)	(61)	(7)
Net cash provided by (used in) investing activities	7,853	88,960	(10,784)
Cash Flows Provided by Financing Activities
Proceeds from issuance of common stock upon public offering, net		33,464	53,760
Proceeds from issuance of common stock through at-the-market offerings, net	10,664	7,397	43,926
Issuance costs related to the issuance of convertible preferred stock	(173)
Proceeds from stock option exercises and employee stock plan purchases	177	1,200	1,222
Net cash provided by financing activities	10,668	42,061	98,908
Net (decrease) increase in cash, cash equivalents, and restricted cash	(47,779)	46,709	9,379
Cash, cash equivalents, and restricted cash at beginning of period	107,586	60,877	51,498
Cash, cash equivalents, and restricted cash at end of period	$ 59,807	$ 107,586	$ 60,877