Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities
Net loss	$ (29,360)	$ (31,194)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities
Depreciation and amortization	1,020	915
Net amortization of premiums and discounts on marketable securities	160	(451)
Stock-based compensation	2,563	1,449
Loss on sale of property and equipment	2	14
Realized gain on marketable securities	(3)	(7)
Changes in assets and liabilities
Prepaid expenses and other assets	(81)	(2,071)
Right-of-use assets	(459)	870
Accounts payable	684	(153)
Accrued and other liabilities	1,186	1,000
Contract liabilities	91,033
Lease liabilities	314	(979)
Net cash provided by (used in) operating activities	67,059	(30,607)
Cash flows from investing activities
Purchases of property and equipment, net	(314)	(1,137)
Purchases of marketable securities	(214,412)	(86,117)
Maturities of marketable securities	73,037	67,538
Sales of marketable securities		18,094
Net cash used in investing activities	(141,689)	(1,622)
Cash flows from financing activities
Proceeds from issuance of common stock upon 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 exercise of common stock options, net of repurchases	1,017	251
Proceeds from ESPP purchase	132
Net cash provided by financing activities	121,801	50,572
Net increase in cash, cash equivalents and restricted cash	47,171	18,343
Cash, cash equivalents and restricted cash, at beginning of period	34,173	20,611
Cash, cash equivalents and restricted cash, at end of period	81,344	38,954
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	81,238	38,848
Restricted cash	106	106
Cash, cash equivalents and restricted cash, at end of period	81,344	38,954
Supplemental disclosure of cash flow information:
Cash paid for income taxes	1	1
Cash paid for interest	62	69
Supplemental non-cash investing and financing activities:
Vesting of early exercised options and restricted stock	44	91
Purchases of property and equipment in accounts payable and accrued liabilities	161	11
Unpaid offering costs	$ 31
Conversion of redeemable convertible preferred stock into common stock		$ 138,391