Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net loss	$ (34,495)	$ (41,975)	$ (34,346)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities
Depreciation and amortization	1,381	1,245	886
Net amortization (accretion) of premiums (discounts) on marketable securities	578	(473)	(808)
Stock-based compensation	3,609	2,168	950
Issuance of Series B redeemable convertible preferred stock pursuant to license agreement			3,812
Change in fair value of redeemable convertible preferred stock liability			(70)
Landlord contributions for leasehold improvements			367
Loss (gain) on sale of property and equipment	2	14	(54)
Gain on sale of marketable securities	(18)	(8)	(11)
Changes in assets and liabilities
Accounts receivable	(1,877)
Prepaid expenses and other assets	(510)	(1,963)	(243)
Right-of-use assets	(148)	1,093
Accounts payable	225	(195)	322
Accrued and other liabilities	2,992	2,030	1,672
Contract liabilities	83,773
Deferred rent			(97)
Lease liabilities	(49)	(1,249)
Net cash provided by (used in) operating activities	55,463	(39,313)	(27,620)
Cash flows from investing activities
Purchases of property and equipment, net	(493)	(1,353)	(1,708)
Purchases of marketable securities	(242,314)	(88,004)	(133,301)
Maturities of marketable securities	96,564	73,528	65,830
Sales of marketable securities		18,094	6,000
Net cash (used in) provided by investing activities	(146,243)	2,265	(63,179)
Cash flows from financing activities
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs			105,358
Proceeds from issuance of common stock in public offering, net of issuance costs	100,664	50,321
Proceeds from issuance of common stock in private placement, net of issuance costs	19,988
Proceeds from issuance of common stock related to at-the-market offering program, net of issuance costs	6,582
Proceeds from exercise of common stock options, net of repurchases	1,190	289	91
Proceeds from ESPP purchases	326
Payments of deferred offering costs			(71)
Net cash provided by financing activities	128,750	50,610	105,378
Net increase in cash, cash equivalents and restricted cash	37,970	13,562	14,579
Cash, cash equivalents and restricted cash, at beginning of period	34,173	20,611	6,032
Cash, cash equivalents and restricted cash, at end of period	72,143	34,173	20,611
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	72,037	34,067	20,505
Restricted cash	106	106	106
Cash, cash equivalents and restricted cash, at end of period	72,143	34,173	20,611
Supplemental disclosure of cash flow information:
Cash paid for income taxes	1	1	1
Cash paid for interest	82	91	99
Supplemental non-cash investing and financing activities:
Right-of-use asset obtained in exchange for new operating lease liability	1,191
Unpaid deferred offering costs			499
Vesting of early exercised options and restricted stock	56	114	189
Purchases of property and equipment in accounts payable and accrued liabilities	$ 520		604
Extinguishment of redeemable convertible preferred stock liability			3,137
Conversion of redeemable convertible preferred stock into common stock		$ 138,391
Series B Redeemable Convertible Preferred Stock
Supplemental non-cash investing and financing activities:
Issuance of Series B redeemable convertible preferred stock pursuant to license agreement			$ 3,812