Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 20,261	$ 32,892
Marketable securities	39,802	28,590
Prepaid expenses and other current assets	1,581	607
Total current assets	61,644	62,089
Restricted cash	1,305	1,303
Property and equipment, net	3,238	3,387
Right of use assets	5,702
Total assets	71,889	66,779
Current liabilities:
Accounts payable	358	1,416
Accrued expenses and other current liabilities	1,956	3,305
Current portion of loans payable, net	14,330	12,562
Lease liability	563
Deferred rent		207
Total current liabilities	17,207	17,490
Loans payable, net		4,811
Capital leases, net of current portion		30
Leases, net of current portion	8,281	20
Deferred rent, net of current portion		2,931
Warrant liability	14	1
Total liabilities	25,502	25,253
Commitments and contingencies (Note 6)
Stockholders' equity:
Common stock-$0.001 par value; 200,000,000 shares authorized; 92,353,638 and 82,323,413 shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively	92	82
Preferred stock-$0.001 par value; 5,000,000 shares authorized, none issued or outstanding at March 31, 2019 and December 31, 2018
Additional paid-in capital	439,735	429,727
Accumulated deficit	(393,443)	(388,274)
Accumulated other comprehensive loss	3	(9)
Total stockholders' equity	46,387	41,526
Total liabilities and stockholders' equity	$ 71,889	$ 66,779