Income Taxes (Tables)

12 Months Ended

Dec. 31, 2013

Reconciliation of Effective Tax Rate for the Provision for Income Taxes

We did not record a tax provision for the years ending December 31, 2013, 2012 and 2011. The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows:

	Year Ended December 31,
	2013			2012			2011
Federal statutory income tax rate		34.0	%		34.0	%		34.0	%
State income taxes, net of federal benefit		0.4			22.8			(3.8	)
Federal and state research credits		3.4			0.8			(5.7	)
Stock based compensation		(0.2	)		0.4			1.3
Other		(0.5	)		(0.1	)		0.6
Change in valuation allowance		(37.1	)		(57.9	)		(26.4	)
		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,
	2013			2012
Deferred tax assets:
Federal and state net operating loss carryforwards	$	102,478		$	74,390
Federal and state research tax credit carryforwards		10,750			7,551
Deferred revenue		1,755			1,434
Stock options		2,895			2,362
Capitalized acquisition costs		1,295			1,503
Other		654			1,859
Total deferred tax assets		119,827			89,099
Valuation allowance		(119,827	)		(89,099	)
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,
	2013			2012		2011
Unrecognized tax benefits, beginning of period	$	1,435		$	1,344	$	973
Gross increases - current period tax positions		619			91		371
Gross decreases - tax position in prior period		(6	)		-		-
Unrecognized tax benefits, end of period	$	2,048		$	1,435	$	1,344