Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating activities
Net loss	$ (23,301)	$ (28,922)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,132	2,846
Depreciation and amortization	63	48
Amortization expense component of lease expense	316	160
Amortization/(accretion) of available-for-sale marketable securities, net	194	(65)
Realized gain on sale of available-for-sale marketable securities	(39)
Realized gain on sale of property and equipment	(70)
Deferred revenue		(8,021)
Changes in operating assets and liabilities:
Income tax receivable		(122)
Other receivables	(1,687)	124
Prepaid expenses	(942)	(1,825)
Accounts payable and accrued expenses	(2,001)	(2,294)
Operating lease liabilities	(386)	(232)
Net cash used in operating activities	(23,721)	(38,303)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	27,655	55,475
Proceeds from redemptions of available-for-sale marketable securities, at par	2,100	2,500
Proceeds from sale of available-for-sale marketable securities	8,029
Purchases of available-for-sale marketable securities	(23,985)	(21,016)
Proceeds from sale of property and equipment	70
Net cash provided by investing activities	13,869	36,959
Financing activities
Proceeds from the exercise of stock options	688	75
Net cash provided by financing activities	688	75
Net decrease in cash, cash equivalents and restricted cash	(9,164)	(1,269)
Cash, cash equivalents and restricted cash at beginning of period	32,091	18,713
Cash, cash equivalents and restricted cash at end of period	$ 22,927	$ 17,444