Income Taxes (Tables)

12 Months Ended

Dec. 31, 2012

Significant Components of Deferred Tax Assets

Significant components of deferred tax assets are as follows:

	December 31,
	2012			2011
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	59,124		$	48,901
Deferred revenue		—			14
Stock-based compensation		2,183			1,306
Research and development tax credits		3,775			3,560
Investment in Partnership		1,389			547
Other		1,804			303
		68,275			54,631
Less valuation allowance		(68,275	)		(54,631	)
Net deferred tax assets	$	—		$	—

Reconciliation of Federal Statutory Tax Rate to Effective Income Tax

A reconciliation of the Federal statutory tax rate of 34% to our effective income tax rate follows:

	December 31,
	2012			2011			2010
Statutory tax rate		(34	)%		(34	)%		(34	)%
Permanent difference		2			1			(1	)
Change in valuation allowance		35			33			27
Other		(3	)		—			8
Effective tax rate		—			—			—