Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Operating activities
Net loss	$ (54,046)	$ (53,990)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	7,608	7,465
Depreciation and amortization	124	96
Amortization expense component of lease expense	638	325
Amortization/(accretion) of available-for-sale marketable securities, net	384	(34)
Realized gain on sale of available-for-sale marketable securities	(39)	(60)
Realized gain on sale of property and equipment	(70)
Deferred revenue		(12,494)
Changes in operating assets and liabilities:
Income tax receivable	810	(304)
Other receivables	252	485
Prepaid expenses	3,781	(1,261)
Accounts payable and accrued expenses	(3,388)	(5,713)
Operating lease liabilities	(780)	(470)
Net cash used in operating activities	(44,726)	(65,955)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	80,470	97,645
Proceeds from redemptions of available-for-sale marketable securities, at par	8,600	17,035
Proceeds from sale of available-for-sale marketable securities	9,029	10,677
Purchases of available-for-sale marketable securities	(63,772)	(21,016)
Proceeds from sale of property and equipment	70
Net cash provided by investing activities	34,397	104,341
Financing activities
Proceeds from the exercise of stock options	981	276
Net cash provided by financing activities	981	276
Net (decrease) increase in cash, cash equivalents and restricted cash	(9,348)	38,662
Cash, cash equivalents and restricted cash at beginning of period	32,091	18,713
Cash, cash equivalents and restricted cash at end of period	$ 22,743	$ 57,375