Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 43,930	$ 43,224
Short-term investments	70,458	83,438
Prepaid expenses and other current assets	2,056	3,002
Restricted cash	0	197
Total current assets	116,444	129,861
Property and equipment, net	5,572	8,313
Investment in joint venture	65	0
Restricted cash	439	439
Other long-term assets	23	0
Total assets	122,543	138,613
Current liabilities:
Accounts payable	2,183	3,352
Accrued expenses and other current liabilities	11,026	7,891
Total current liabilities	13,209	11,243
Other non-current liabilities	1,116	520
Total liabilities	14,325	11,763
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; 35,641,505 and 27,296,747 shares issued and outstanding at December 31, 2016 and 2015, respectively	36	27
Additional paid-in capital	358,419	294,910
Accumulated other comprehensive loss	(60)	(170)
Accumulated deficit	(250,177)	(167,917)
Total stockholders' equity	108,218	126,850
Total liabilities and stockholders' equity	$ 122,543	$ 138,613