Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Tax Provision

The income tax provision consists of the following (in thousands):

	Year Ended December 31,
	2018		2017
Current:
Federal	$	–	$	–
State		–		–
Foreign		–		–
		–		-
Deferred:
Federal	$	–	$	–
State		–		–
Foreign		–		–
		–		–
Total provision for income taxes	$	-	$	-

Reconciliation of Effective Tax Rate for the Provision for Income Taxes

The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows:

	Year Ended December 31,
	2018			2017			2016
Federal statutory income tax rate		21.0	%		34.0	%		34.0	%
Federal and state credits		7.4	%		8.9	%		9.5	%
Excess tax benefit		0.4	%		8.6	%		0.0	%
Stock based compensation		-0.8	%		0.1	%		-0.1	%
Other		-0.6	%		-0.1	%		0.1	%
Tax impact due to tax rate reduction		0.0	%		-47.7	%		0.0	%
Change in valuation allowance		-24.1	%		2.6	%		-38.6	%
Foreign Rate Differential		-3.3	%		-6.4	%		-4.9	%
Total tax benefit		0.0	%		0.0	%		0.0	%

Components of U.S. Deferred Tax Assets and (Liabilities)

The components of U.S. deferred tax assets and (liabilities) are as follows (in thousands):

	December 31,
	2018			2017
Deferred tax assets:
Federal and state net operating loss carryforwards	$	241,881		$	203,897
Federal and state research tax credit carryforwards		23,917			21,238
Federal Orphan Drug Credit		120,273			96,750
Deferred revenue		29,530			17,538
Stock options		19,992			15,995
Other		9,448			7,529
Net deferred tax assets before valuation allowance		445,041			362,947
Valuation allowance		(445,041	)		(362,947	)
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,
	2018			2017			2016
Unrecognized tax benefits, beginning of period	$	20,730		$	13,865		$	3,228
Increases due to current period positions		4,879			7,046			6,919
Increase due to prior period positions		26			–			4,266
Decreases due to prior period positions		(214	)		(181	)		(548	)
Unrecognized tax benefits, end of period	$	25,421		$	20,730		$	13,865