CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities
Net loss	$ (47,329)	$ (51,611)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	960	1,353
Amortization of deferred financing costs	282	282
Amortization of deferred compensation for services	158	154
Amortization of premium on investment securities	(227)	(537)
Non-cash lease expense	1,025	891
Stock-based compensation	5,622	3,845
Change in derivative liabilities	152	61
Non-cash interest associated with debt discount accretion	257	231
Changes in operating assets and liabilities:
Unbilled revenue		5,000
Prepaid expenses and other assets	(2,144)	12
Accounts payable	2,025	(887)
Accrued compensation and benefits	(1,372)	(372)
Lease liabilities	(1,251)	(825)
Accrued and other liabilities	174	(2,330)
Deferred revenue	(2,300)
Net cash used in operating activities	(43,968)	(44,733)
Investing activities
Proceeds from maturities of investments	25,519	86,454
Purchases of investments	(62,960)	(30,857)
Purchases of property and equipment	(25)	(211)
Net cash (used in) provided by investing activities	(37,466)	55,386
Financing activities
Proceeds from issuance of common stock under stock plans	375	198
Issuance of common stock upon exercise of options	420	7
Net cash provided by financing activities	795	205
Net (decrease) increase in cash and cash equivalents	(80,639)	10,858
Cash and cash equivalents at beginning of period	181,133	78,768
Cash and cash equivalents at end of period	$ 100,494	89,626
Supplementary disclosure of cash flow information:
Income taxes paid		2
Supplementary disclosure of non-cash activities:
Right-of-use assets obtained in exchange for lease obligations		$ 5,810