Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

Income Taxes

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

	December 31,
	2016				2015
	(in thousands, except percentages)
U.S. federal taxes (benefit) at statutory rate	$	(32,277)	(34.0)	%	$	(24,375)	(34.00)	%
State tax expense		(2,842)	(2.99)	%		(2,428)	(3.39)	%
Research and development credits		(1,449)	(1.53)	%		(751)	(1.05)	%
Stock-based compensation		1,275	1.34	%		1,683	2.35	%
Mark to market fair value adjustments		49	0.05	%		504	0.70	%
Other nondeductible items		933	0.99	%		841	1.17	%
Change in valuation allowance		34,453	36.29	%		24,526	34.21	%
Provision for income taxes	$	142	0.15	%	$	—	—	%

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

	December 31,
	2016		2015
	(in thousands)
Deferred tax assets
Net operating loss carryforwards	$	68,504	$	41,451
Research and development tax credit carryforwards		8,634		4,794
Reserves and accruals		5,719		3,108
Stock-based compensation		3,220		2,206
Total deferred tax assets before valuation allowance		86,077		51,559
Less: valuation allowance		(85,606)		(51,153)
		471		406
Deferred tax liabilities
Property and equipment		(471)		(406)
Net deferred tax assets	$	—	$	—

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

	December 31,
	2016		2015
	(in thousands)
Balance at beginning of year	$	2,405	$	1,360
Additions based on tax positions related to the current year		1,836		1,045
Additions for tax positions of prior years		52		—
Balance at end of year	$	4,293	$	2,405