Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 50,218	$ 73,662
Accounts receivable	288	369
Prepaid expenses and other current assets	562	838
Inventories	1,435	683
Total current assets	52,503	75,552
Property and equipment, net	12,815	10,655
Restricted cash	260	260
Other assets	331	358
Total assets	65,909	86,825
Current liabilities:
Accounts payable	795	1,228
Accrued expenses and other current liabilities	4,506	4,162
Current portion of notes payable	10,631	4,449
Deferred revenue	1,266	2,146
Current portion of lease incentives	284	268
Total current liabilities	17,482	12,253
Notes payable, net of current portion	24,168	26,121
Lease incentives, net of current portion	935	1,076
Other liabilities	665	436
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued and outstanding at June 30, 2016 and December 31, 2015
Common stock, $0.001 par value; 200,000,000 shares authorized at June 30, 2016 and December 31, 2015; 24,364,743 and 24,175,381 shares issued and outstanding at June 30, 2016 and December 31, 2015, respectively	24	24
Additional paid-in capital	198,992	195,800
Accumulated deficit	(176,357)	(148,885)
Total stockholders' equity	22,659	46,939
Total liabilities and stockholders' equity	$ 65,909	$ 86,825