Income Taxes (Tables)

12 Months Ended

Dec. 31, 2021

Income Tax Disclosure [Abstract]

Summary of Income (Loss) Before Income Tax Expense

Income (loss) before income tax expense consists of the following:

	Year Ended December 31,
	2021			2020			2019
	(in thousands)
Domestic	$	(457,693	)	$	639,986		$	(634,289	)
Foreign		(199	)		(33,913	)		(45,949	)
	$	(457,892	)	$	606,073		$	(680,238	)

Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate

A reconciliation of the U.S. statutory rate to the Company’s effective tax rate is as follows:

	Year Ended December 31,
	2021			2020			2019
Tax due at statutory rate		21.0	%		21.0	%		21.0	%
State taxes, net of federal		1.9			5.7			2.8
Biogen transaction-related items		—			(10.1	)		—
Stock-based compensation		(5.2	)		0.9			1.4
Foreign rate differential		—			1.2			(1.6	)
Federal and state credits		3.5			(1.5	)		2.4
Change in valuation allowance		(20.7	)		(17.6	)		(25.8	)
Other		(0.5	)		0.4			(0.2	)
		0.0	%		0.0	%		0.0	%

Summary of Significant Components of Company's Net Deferred Tax Asset

Significant components of the Company’s net deferred tax assets at December 31, 2021 and 2020 are as follows:

	December 31,
	2021			2020
	(in thousands)
Net operating losses	$	305,824		$	222,607
Capitalized start-up costs		735			873
Tax credit carryforwards		106,176			90,460
Accrued expenses		6,684			3,157
Depreciation and amortization		1,401			1,614
Stock-based compensation		49,281			56,520
Right of use asset		(3,702	)		(5,981	)
Lease liability		4,236			6,705
Other		(31	)		148
Total net deferred tax asset before valuation    allowance		470,604			376,103
Valuation allowance		(470,604	)		(376,103	)
Net deferred tax asset	$	—		$	—

Summary of Gross Unrecognized Tax Benefits Excluding Interest and Penalties

The following table reconciles the beginning and ending amounts of gross unrecognized tax benefits, excluding interest and penalties, if any, for the year ended December 31, 2021:

	2021
	(in thousands)
Balance as of January 1	$	-
Increases related to current year tax positions		396
Increases related to prior year tax positions		5,688
Balance as of December 31	$	6,084