For the years ended December 31, 2020, 2019 and 2018, we did not record a provision for income taxes due to a full valuation allowance against our deferred tax assets.

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,
	2020			2019			2018
Tax at statutory federal rate	$	(47,728	)	$	(42,997	)	$	(37,557	)
State tax, net of federal benefit		(8,218	)		(9,823	)		(6,527	)
Research and development credits		(4,327	)		(4,855	)		(4,775	)
Stock-based compensation expense		4,675			2,906			(2,059	)
Non-deductible compensation		1,455			4,720			901
Change in valuation allowance		53,621			49,479			50,834
Other		522			570			(817	)
Provision for income taxes	$	—		$	—		$	—

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,
	2020			2019
Deferred tax assets:
Net operating loss carryforward	$	251,971		$	210,301
Research and development credits		49,683			42,490
Stock-based compensation		20,211			15,628
Lease liabilities		4,330			3,487
Other		3,777			3,818
Total gross deferred tax assets		329,972			275,724
Deferred tax liabilities:
Right-of-use lease assets		(4,014	)		(3,385	)
Total gross deferred tax liabilities		(4,014	)		(3,385	)
Valuation allowance		(325,958	)		(272,339	)
Net deferred tax assets	$	—		$	—

We have established a valuation allowance to offset net deferred tax assets as of December 31, 2020 and 2019 due to the uncertainty of realizing future tax benefits from such assets.

As of December 31, 2020, we had federal, California and other state net operating loss (“NOL”) carryforwards of $1.0 billion, $111.9 million and $515.5 million, respectively. The federal NOL carryforwards consist of $549.7 million generated before January 1, 2018, which will begin to expire in 2021, and $492.0 million that can be carried forward indefinitely, but are subject to the 80% taxable income limitation. The state NOL carryforwards will begin to expire in 2028.

As of December 31, 2020, we had federal and state research and development credit carryforwards of $39.4 million and $18.1 million, respectively. The federal research and development credit carryforwards will begin to expire in 2022. The state research and development credit carryforwards will begin to expire in 2023.

Internal Revenue Code (“IRC”) Sections 382 and 383 place a limitation on the amount of taxable income that can be offset by NOL and credit carryforwards after a change in control (generally greater than 50% change in ownership within a three-year period) of a loss corporation. Generally, after a change in control, a loss corporation cannot deduct NOL and credit carryforwards in excess of the IRC Sections 382 and 383 limitation. State jurisdictions have similar rules. We have previously performed an analysis of IRC Sections 382 and 383 through 2018 and determined there were ownership changes in 2007, 2011 and 2013. We are currently in the process of updating our IRC Sections 382 and 383 analysis through 2020. The limitation in the federal and state NOL and research and development credit carryforwards that expire unused would reduce the deferred tax assets, which are fully offset by a valuation allowance.

We file U.S. and state income tax returns with varying statutes of limitations. The tax years from 2002 to 2020 remain open to examination due to the carryover of unused NOL carryforwards and tax credits.

	Year Ended December 31,
	2020		2019		2018
Balance at beginning of year	$	4,784	$	2,385	$	120
Additions for tax positions of prior years		341		147		—
Additions based on tax positions related to current year		2,281		2,252		2,265
Balance at end of year	$	7,406	$	4,784	$	2,385

Due to our valuation allowance, the $7.4 million of unrecognized tax benefits would not affect the effective tax rate, if recognized. It is the Company’s practice to recognize interest and penalties related to income tax matters in income tax expense. As of December 31, 2020, we had no accrued interest and penalties related to uncertain tax positions. We do not expect any material changes to the estimated amount of liability associated with our uncertain tax positions within the next 12 months.

On March 27, 2020, the Coronavirus Aid, Relief and Economic Security Act (“CARES Act”) was enacted and signed into law in response to the COVID-19 pandemic. GAAP requires recognition of the tax effects of new legislation during the reporting period that includes the enactment date. The CARES Act includes changes to the tax provisions that benefits business entities and makes certain technical corrections to the 2017 Tax Cuts and Jobs Act. The tax relief measures for businesses include a five-year net operating loss carryback, suspension of the annual deduction limitation of 80% of taxable income from net operating losses generated in a tax year beginning after December 31, 2017, changes in the deductibility of interest, acceleration of alternative minimum tax credit refunds, payroll tax relief, technical corrections on net operating loss carryforwards for fiscal year taxpayers and allows accelerated deduction qualified improvement property. The CARES Act also provides other non-tax benefits to assist those impacted by the pandemic. We evaluated the impact of the CARES Act and determined that there was no material impact for the year ended December 31, 2020.

On June 29, 2020, California Assembly Bill 85 was signed into law. The legislation suspends the California net operating loss deductions for 2020, 2021, and 2022 for certain taxpayers and imposes a limitation of certain California tax credits for 2020, 2021, and 2022. The legislation disallows the use of California net operating loss deductions if the taxpayer recognizes business income and its adjusted gross income is greater than $1,000,000. The carryover periods for net operating loss deductions disallowed by this provision will be extended. Additionally, any business credit will only offset a maximum of $5,000,000 of California tax. Given our loss position in the current year, the new legislation did not impact the current year provision or our financial statements for the year ended December 31, 2020. We will continue to monitor possible California net operating loss and credit limitations in future periods.

On December 27, 2020, the “Consolidated Appropriations Act, 2021” was enacted and signed into law to further COVID-19 economic relief and extend certain expiring tax provisions. The relief package includes a tax provision clarifying that businesses with forgiven PPP loans can deduct regular business expenses that are paid for with the loan proceeds. Additional pandemic relief tax measures include an expansion of the employee retention credit, enhanced charitable contribution deductions, and a temporary full deduction for business expenses for food

and beverages provided by a restaurant. The provisions did not have a material impact on our financial statements for the year ended December 31, 2020.