Income Taxes (Tables)

12 Months Ended

Dec. 31, 2015

Income Tax Disclosure [Abstract]

U.S. Income Tax Provision (Benefit)

The U.S. income tax provision (benefit) consists of the following (in thousands):

	Year Ended December 31,
	2015			2014
Current:
Federal	$	–		$	–
State		(365	)	–
		(365	)	–
Deferred:
Federal	$	–		$	–
State	–			–
	–			–
Total provision (benefit) for income taxes	$	(365	)	$	–

Reconciliation of Effective Tax Rate for the Provision for Income Taxes

We did not record a tax provision for the years ended December 31, 2014 and 2013. The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows:

	Year Ended December 31,
	2015			2014			2013
Federal statutory income tax rate		34.0	%		34.0	%		34.0	%
State income taxes, net of federal benefit		(6.6)			11.2			0.4
Federal and state research credits		2.5			2.7			3.4
Stock based compensation		0.0			(1.6	)		(0.2	)
FIN 48 release		0.2			0.0			0.0
Other		0.0			(0.1	)		(0.5	)
Change in valuation allowance		(29.9	)		(46.2	)		(37.1	)
Total tax benefit		0.2	%		0.0	%		0.0	%

Components of U.S. Deferred Tax Assets and (Liabilities)

The components of U.S. deferred tax assets and (liabilities) are as follows (in thousands):

	December 31,
	2015			2014
Deferred tax assets:
Federal and state net operating loss carryforwards	$	207,898		$	146,725
Federal and state research tax credit carryforwards		18,744			15,337
Deferred revenue		9,192			12,523
Stock options		10,197			3,776
Capitalized acquisition costs		974			1,322
Other		3,942			3,589
Net deferred tax assets before valuation allowance		250,947			183,272
Valuation allowance		(250,947	)		(183,272	)
Net deferred tax assets	$	–		$	–

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):

	Year Ended December 31,
	2015			2014		2013
Unrecognized tax benefits, beginning of period	$	2,906		$	2,048	$	1,435
Increases due to current period positions		1,091			858		619
Decreases due to current period positions	–			–		–
Decreases due to prior period positions		(404	)	–			(6	)
Decreases due to the lapse of statutes of limitations		(365	)	–		–
Unrecognized tax benefits, end of period	$	3,228		$	2,906	$	2,048