Income taxes (Tables)

12 Months Ended

Dec. 31, 2012

Components of the net deferred tax asset as of December 31, 2011 and 2012 are approximately as follows (in thousands):


	December 31,
	2011			2012
Deferred tax assets:
Net operating loss carryforwards	$	557,427		$	609,471
Research and development credits		59,848			65,315
Capitalized research		32,440			31,490
Accrued expenses		2,827			2,922
Non-qualified stock option expense		27,964			30,928
Depreciation		8,906			10,025

Total net deferred tax assets		689,412			750,151
Valuation allowance		(689,412	)		(750,151	)

Net deferred tax assets	$	—		$	—

Effective Income Tax Rate

The Company’s effective income tax rate differs from the statutory federal income tax rate as follows for the years ended December 31, 2010, 2011 and 2012:


	December 31,
	2010			2011			2012
Federal tax benefit rate		35.0	%		35.0	%		35.0	%
State tax benefit, net of federal benefit		—			—			—
Permanent items		—			—			—
Intercompany transfer of intellectual property		(5.0	)		(5.0	)		(4.0	)
Valuation allowance		(30.0	)		(30.0	)		(31.0	)

Effective income tax rate		0.0	%		0.0	%		0.0	%