Income Taxes Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

The following table represents the significant components of our deferred tax assets and liabilities for the periods presented:



	December 31,
	2019			2018
Deferred tax assets
Net operating loss carryforwards	$	49,211		$	31,311
Credit carryforwards	8,638			6,655
Stock-based compensation	5,142			4,073
Compensation accruals	2,297			1,873
Lease liability	18,404			—
Accruals and reserves	795			3,223
Gross deferred tax assets	84,487			47,135
Valuation allowance	(62,274		)	(38,010		)
Total deferred tax assets, net of valuation allowance	22,213			9,125
Deferred tax liabilities
Intangible assets	(605		)	(734		)
Prepaid expenses	(1,688		)	(1,503		)
Convertible senior notes	(5,359		)	(6,888		)
Right-of-use assets	(13,579		)	—
Fixed assets	(982		)	—
Total deferred tax liabilities	(22,213		)	(9,125		)
Net deferred tax assets and liabilities	$	—		$	—

The following table represents our net operating loss ("NOL") carryforwards as of December 31, 2019 and 2018:

reconciliation of the U.S. federal income tax at statutory rate to our effective income tax rate:



	December 31,
	2019		2018		2017
U.S. federal income tax at statutory rate	21.00	%	21.00	%	34.00	%
State taxes (net of federal benefit)	4.71		5.67		2.40
Stock-based compensation	1.20		7.51		(14.74	)
Permanent differences	(0.97	)	(0.57	)	(0.29	)
Federal research and development credit	2.45		4.26		7.08
Change in valuation allowance	(29.73	)	(37.33	)	(27.79	)
Other	1.34		(0.54	)	(0.66	)
Change in valuation allowance for Tax Act impact	—		—		84.37
Change in deferred balance before valuation allowance for Tax Reform impact	—		—		(84.37	)
Effective income tax rate	—	%	—	%	—	%

The following table summarizes the activity related to unrecognized tax benefits:



	December 31,
	2019			2018
Unrecognized benefit—beginning of year	$	1,663		$	1,057
Gross decreases—prior year tax positions	(127		)	—
Gross increases—current year tax positions	623			606
Unrecognized benefit—end of year	$	2,159		$	1,663