Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Summary of loss before provision for income tax

Loss before provision for income tax is summarized as follows (in thousands):



	Year Ended December 31,
	2017			2016			2015
United States	$	(91,220	)	$	(60,147	)	$	(57,074	)
International	—			(26		)	—
Total	$	(91,220	)	$	(60,173	)	$	(57,074	)

Summary of income tax provision

The provision for income taxes is summarized as follows (in thousands):



	December 31,
	2017			2016			2015
Current:
Federal	$	(1,730	)	$	(116	)	$	(5,273	)
State	—			1			1
Foreign	—			—			—
	(1,730		)	(115		)	(5,272		)
Deferred:
Federal	—			—			—
State	—			—			—
Foreign	—			—			—
	—			—			—
Benefit for income taxes	$	(1,730	)	$	(115	)	$	(5,272	)

Schedule showing the difference of provision for income taxes from the amount computed by applying the federal income tax rate of 35% to pretax income (loss) from operations

The provision for income taxes differs from the amount computed by applying the federal income tax rate of 35% to pretax loss from operations as a result of the following:



	December 31,
	2017			2016			2015
Statutory federal income tax rate	$	(31,927	)	$	(21,079	)	$	(19,976	)
State income taxes, net of federal tax benefits	(5,041		)	(9		)	1
Foreign rate differential	—			10			—
Tax credits	(2,306		)	(3,905		)	(8,303		)
Impact of federal rate change	24,907			—			—
Net operating loss carryback	—			—			4,099
Change in statutory rates	(1,440		)	624			—
Stock compensation	(2,558		)	(1,109		)	821
State net operating losses	633			1,779			—
Other	—			109			1,330
Change in valuation allowance	16,002			23,465			16,756
Income tax provision	$	(1,730	)	$	(115	)	$	(5,272	)

Schedule of deferred tax assets and liabilities

Significant components of the Company’s deferred tax assets and liabilities are as follows (in thousands):



	December 31,
	2017			2016
Net operating loss carryforwards	$	44,177		$	29,102
Research and development tax credits	16,189			14,453
Accruals and reserves	371			395
Stock compensation	6,244			6,902
Depreciation and amortization	1,638			1,765
Total deferred tax assets	68,619			52,617
Less: Valuation allowance	(68,619		)	(52,617		)
Net deferred tax assets	$	—		$	—

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

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands):



	December 31,
	2017		2016		2015
Balance at the beginning of the year	$	3,188	$	1,820	$	2,608
Additions based on prior period tax positions	43		93		980
Additions based on current period tax positions	803		1,275		—
Reductions based on prior period tax positions	—		—		(1,768		)
Balance at the end of the year	$	4,034	$	3,188	$	1,820