Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 19,721	$ 15,703
Short-term investments	38,577	51,500
Prepaid expenses and other current assets	835	1,578
Total current assets	59,133	68,781
Property and equipment, net	2,935	3,113
Investment in joint venture	146	146
Long-term restricted cash	791	789
Other long-term assets	24	24
Total assets	63,029	72,853
Current liabilities:
Accounts payable	1,024	2,242
Accrued expenses and other current liabilities	4,008	5,562
Total current liabilities	5,032	7,804
Other non-current liabilities	664	751
Total liabilities	5,696	8,555
Commitments and contingencies (Note 10)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized, no shares issued and outstanding	0	0
Common stock, $0.001 par value; 100,000,000 shares authorized; 35,758,907 and 35,725,230 shares issued and outstanding at March 31, 2018 and December 31, 2017, respectively	36	36
Additional paid-in capital	366,178	365,769
Accumulated other comprehensive loss	(31)	(27)
Accumulated deficit	(308,850)	(301,480)
Total stockholders’ equity	57,333	64,298
Total liabilities and stockholders’ equity	$ 63,029	$ 72,853