INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2014

Income Tax Disclosure [Abstract]

Analysis of Deferred Tax Assets (Liabilities)

An analysis of the deferred tax assets (liabilities) is as follows:

	As of December 31,
	2014			2013
	(in thousands)
Net operating loss carryforwards	$	115,699		$	94,170
Difference in depreciation and amortization		2,800			2,492
Research and development tax credits		29,127			23,599
Stock-based compensation		13,637			9,036
Deferred rent		2,864			2,849
Deferred revenue		1,213			1,219
Other		413			208
Gross deferred tax assets		165,753			133,573
Valuation allowance		(165,753	)		(133,573	)
Net deferred tax asset	$	—		$	—

Reconciliation Between Effective Tax Rate and Income Tax Rate

The reconciliation between the Company’s effective tax rate and the income tax rate is as follows:

	For the Year Ended December 31,
	2014			2013			2012
Federal income tax rate		34.0	%		34.0	%		34.0	%
Research and development tax credits		2.4			1.4			(0.6	)
Valuation allowance		(21.6	)		(12.4	)		(7.5	)
Permanent Differences		(2.4	)		(8.8	)		(25.9	)
Foreign rate differential		(12.4	)		(14.2	)		—
Effective tax rate		—	%		—	%		—	%