Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating activities
Net loss	$ (28,922)	$ (21,960)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,846	2,234
Depreciation and amortization	48	50
Amortization expense component of lease expense	160	145
Accretion of available-for-sale marketable securities, net	(65)	(478)
Deferred revenue	(8,021)	(4,242)
Changes in operating assets and liabilities:
Income tax receivable	(122)	(85)
Other receivables	124	(93)
Prepaid expenses	(1,825)	(2,574)
Accounts payable and accrued expenses	(2,294)	(312)
Operating lease liability	(232)	(211)
Net cash used in operating activities	(38,303)	(27,526)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	55,475	79,295
Proceeds from the redemptions of available-for-sale marketable securities, at par	2,500
Purchases of available-for-sale marketable securities	(21,016)	(52,885)
Purchases of property and equipment		(11)
Net cash provided by investing activities	36,959	26,399
Financing activities
Proceeds from the exercise of stock options	75	234
Net cash provided by financing activities	75	234
Net decrease in cash, cash equivalents and restricted cash	(1,269)	(893)
Cash, cash equivalents and restricted cash at beginning of period	18,713	15,850
Cash, cash equivalents and restricted cash at end of period	$ 17,444	14,957
Noncash investing and financing activities
Shares of common stock issued in exchange for consulting services		$ 197