Income Taxes (Tables)

12 Months Ended

Dec. 31, 2021

Income Tax Disclosure [Abstract]

Schedule of Components of Income Tax Expense (Benefit)

The difference between the provision for income taxes and income taxes computed using the effective U.S. federal statutory rate is as follows (in thousands):

	December 31,
	2021			2020			2019
Tax at statutory federal rate	$	(46,343	)	$	(47,728	)	$	(42,997	)
State tax, net of federal benefit		(8,683	)		(8,218	)		(9,823	)
Research and development credits		(4,173	)		(4,327	)		(4,855	)
Stock-based compensation expense		4,584			4,675			2,906
Non-deductible compensation		1,970			1,455			4,720
Change in valuation allowance		51,459			53,621			49,479
Other		1,186			522			570
Provision for income taxes	$	—		$	—		$	—

Schedule of Deferred Tax Assets and Liabilities

Deferred income tax assets and liabilities arising from differences between accounting for financial statement purposes and tax purposes, less valuation allowance at year-end are as follows (in thousands):

	December 31,
	2021			2020
Deferred tax assets:
Net operating loss carryforward	$	294,189		$	251,971
Research and development credits		55,789			49,683
Stock-based compensation		23,342			20,211
Lease liabilities		2,564			4,330
Other		3,953			3,777
Total gross deferred tax assets		379,837			329,972
Deferred tax liabilities:
Right-of-use lease assets		(2,420	)		(4,014	)
Total gross deferred tax liabilities		(2,420	)		(4,014	)
Valuation allowance		(377,417	)		(325,958	)
Net deferred tax assets	$	—		$	—

Summary of Unrecognized Tax Benefits

A reconciliation of our unrecognized tax benefits is as follows (in thousands):

	Year Ended December 31,
	2021		2020		2019
Balance at beginning of year	$	7,406	$	4,784	$	2,385
Additions for tax positions of prior years		107		341		147
Additions based on tax positions related to current year		2,118		2,281		2,252
Balance at end of year	$	9,631	$	7,406	$	4,784