Income Taxes (Tables)

12 Months Ended

Dec. 31, 2014

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, 2013 and 2012. The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows:

	Year Ended December 31,
	2014			2013			2012
Federal statutory income tax rate		34.0	%		34.0	%		34.0	%
State income taxes, net of federal benefit		11.2			0.4			22.8
Federal and state research credits		2.7			3.4			0.8
Stock based compensation		(1.6	)		(0.2	)		0.4
Other		(0.1	)		(0.5	)		(0.1	)
Change in valuation allowance		(46.2	)		(37.1	)		(57.9	)
		0.0	%		0.0	%		0.0	%

Significant Components of Deferred Tax Assets

Significant components of our deferred tax assets are as follows (in thousands):

	December 31,
	2014			2013
Deferred tax assets:
Federal and state net operating loss carryforwards	$	146,725		$	102,478
Federal and state research tax credit carryforwards		15,337			10,750
Deferred revenue		12,523			1,755
Stock options		3,776			2,895
Capitalized acquisition costs		1,322			1,295
Other		3,589			654
Total deferred tax assets		183,272			119,827
Valuation allowance		(183,272	)		(119,827	)
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,
	2014		2013			2012
Unrecognized tax benefits, beginning of period	$	2,048	$	1,435		$	1,344
Gross increases - current period tax positions		858		619			91
Gross decreases - tax position in prior period		–		(6	)		–
Unrecognized tax benefits, end of period	$	2,906	$	2,048		$	1,435