STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows from Operating Activities:
Net loss	$ (112,662)	$ (56,515)	$ (59,881)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	3,641	3,845	5,674
Warrant liability revaluation loss	3,742	598
Future tranche right liability revaluation loss	72,367	10,964
Issuance of common stock for services rendered	243	129	97
Accretion of discounts on short-term investments	(46)	(372)
Depreciation and amortization expense	61	120	432
(Gain) loss on disposal of property and equipment		(10)	477
Changes in operating assets and liabilities:
Prepaid expenses and other assets	500	(2,160)	2,717
Accounts payable, accrued expenses, and other liabilities	(1,629)	(1,105)	(866)
Deferred revenue			(566)
Other	11	8
Net cash used in operating activities	(33,772)	(44,498)	(51,916)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities	(12,178)	(44,502)
Proceeds from maturity of available-for-sale securities	10,499	42,100
Proceeds from the sale of property and equipment		11	290
Purchases of property and equipment	(8)	(11)	(75)
Net cash (used in) provided by investing activities	(1,687)	(2,402)	215
Cash Flows from Financing Activities:
Proceeds from private placement		10,072
Proceeds from common stock financings, net	28,758	5,298
Proceeds from employee stock purchases	113	121	243
Proceeds from exercise of common stock options and warrants	15	3	10,166
Payments on note payable and seller-financed purchases	(217)		(209)
Other		(6)	(8)
Net cash provided by financing activities	28,669	15,488	10,192
Net decrease in cash and cash equivalents	(6,790)	(31,412)	(41,509)
Cash and cash equivalent, beginning of period	40,019	71,431	112,940
Cash and cash equivalents, end of period	33,229	40,019	71,431
Supplemental disclosure of cash flow information:
Cash paid for interest	3		9
Increase to operating lease right-of-use asset upon adoption of ASC 842		1,236
Increase to operating lease right-of-use assets upon acquisition	54
Increase to lease liability upon adoption of ASC 842		1,236
Increase to operating lease liability upon acquisition	54
Supplemental disclosure of non-cash financing and investing activities:
Offering costs in accounts payable and accrued expenses	112	$ 165	$ 101
Non-cash seller-financed purchases	$ 652