Income Taxes (Tables)

12 Months Ended

Dec. 31, 2015

A reconciliation of the Company’s effective tax rate to the statutory federal income tax rate is as follows:


	Years Ended December 31,
	2015			2014			2013
Federal income tax at statutory federal rate		35.0	%		35.0	%		35.0	%
State taxes		1.0			3.1			4.3
Permanent differences		(6.4	)		(9.1	)		(2.0	)
Stock-based compensation		(1.3	)		(1.7	)		(0.6	)
Tax credits		21.3			30.5			12.4
Foreign rate differential		—			(2.3	)		(2.8	)
Change in deferred state tax rate		(2.3	)		—			—
Other		(0.5	)		4.5			(1.5	)
Change in valuation allowance		(46.8	)		(60.0	)		(44.8	)

		—	%		—	%		—	%

Temporary differences that give rise to significant net deferred tax assets as of December 31, 2015 and 2014 are as follows:


(in thousands)	December 31, 2015			December 31, 2014
Deferred tax assets
Net operating losses	$	182,992		$	160,333
Capitalized research and development expenses		21,444			30,929
Credit carryforwards		93,113			62,362
Depreciation		2,128			2,579
Deferred compensation		11,664			10,682
Accrued expenses		1,807			12
Deferred revenue		10,999			960
Other temporary differences		17,235			9,195

Total gross deferred tax asset		341,382			277,052
Valuation allowance		(326,577	)		(257,489	)

Net deferred tax asset		14,805			19,563
Deferred tax liabilities
Intangible assets		(2,667	)		(3,106	)
Debt discount		(12,138	)		(16,457	)

Net deferred taxes	$	—		$	—

The change in the valuation allowance against the deferred tax assets in the years ended December 31, 2015, 2014 and 2013 was as follows:


(in thousands)	Balance at Beginning of Period		Additions		Deductions		Balance at End of Period
December 31, 2013	$	169,651	$	37,653	$	—	$	207,304
December 31, 2014		207,304		50,185		—		257,489
December 31, 2015		257,489		69,088		—		326,577