Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 28,090	$ 32,305
Marketable securities		3,500
Prepaid expenses and other current assets	1,666	1,683
Total current assets	29,756	37,488
Restricted cash	1,310	1,309
Property and equipment, net	2,577	2,705
Right-of-use lease asset	5,389	5,472
Other assets	18	20
Total assets	39,050	46,994
Current liabilities:
Accounts payable, net	698	1,047
Accrued expenses and other current liabilities	1,277	2,403
Current portion of loans payable, net		5,037
Lease liability	642	620
Total current liabilities	2,617	9,107
Leases, net of current portion	7,636	7,804
Warrant liability	2	5
Total liabilities	10,255	16,916
Commitments and contingencies (Note 8)
Stockholders' equity:
Common stock?$0.001 par value; 200,000,000 shares authorized at March 31, 2020 and December 31, 2019; 99,030,004 and 94,213,760 shares issued and outstanding at March 31, 2020 and December 31, 2019, respectively	99	94
Preferred stock?$0.001 par value; 5,000,000 shares authorized, none issued or outstanding at March 31, 2020 and December 31, 2019
Additional paid-in capital	447,566	443,129
Accumulated deficit	(418,870)	(413,145)
Total stockholders' equity	28,795	30,078
Total liabilities and stockholders' equity	$ 39,050	$ 46,994