Income Taxes (Tables)

12 Months Ended

Dec. 31, 2015

Income Taxes

Schedule of effective tax rates differing from U.S. federal statutory rate

	December 31,
	2015				2014
	(in thousands, except percentages)
U.S. federal taxes (benefit) at statutory rate	$	(24,375)	(34.00)	%	$	(1,747)	(34.00)	%
State tax expense		(2,428)	(3.39)	%		(294)	(5.72)	%
Research and development credits		(751)	(1.05)	%		(530)	(10.32)	%
Stock-based compensation		1,683	2.35	%		755	14.70	%
Mark to market fair value adjustments		504	0.70	%		566	11.01	%
Other nondeductible items		841	1.17	%		414	8.05	%
Change in valuation allowance		24,526	34.21	%		851	16.57	%
Provision for income taxes	$	—	—	%	$	15	0.29	%

Schedule of tax effects of temporary differences that give rise to significant portions of the deferred tax assets

	December 31,
	2015		2014
	(in thousands)
Deferred tax assets
Net operating loss carryforwards	$	41,451	$	21,587
Research and development tax credit carryforwards		4,794		2,696
Reserves and accruals		3,108		1,717
Stock-based compensation		2,206		1,505
Total deferred tax assets before valuation allowance		51,559		27,505
Less: valuation allowance		(51,153)		(26,627)
		406		878
Deferred tax liabilities
Property and equipment		(406)		(878)
Net deferred tax assets	$	—	$	—

Schedule of reconciliation of the beginning and ending amount of gross unrecognized tax benefits

	December 31,
	2015		2014
	(in thousands)
Balance at beginning of year	$	1,360	$	898
Additions based on tax positions related to the current year		1,045		462
Additions for tax positions of prior years		—		—
Balance at end of year	$	2,405	$	1,360