The following table reconciles the federal statutory tax rate to the effective income tax rate from continuing operations:

	2020				2019				2018
Tax at statutory rate		21.0		%		21.0		%		21.0		%
State income tax, net of federal benefit		6.9				12.2				(1.4	)
Federal and state tax credits		5.3				4.0				3.9
Stock-based compensation		(0.5	)			(0.8	)			2.1
Net operating loss not benefitted		(6.9	)			(5.8	)			(4.3	)
Other		(0.3	)			(0.2	)			(5.5	)
Change in valuation allowance		(25.5	)			(30.4	)			(15.8	)
Effective tax rate		0.0		%		0.0		%		0.0		%

Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of our deferred tax assets are as follows:

	December 31,
	2020			2019
	(In thousands)
Net operating loss carryforwards	$	217,100		$	204,600
Federal and state tax credits		42,800			38,400
Capitalized research and development		6,000			5,900
Stock-based compensation		9,400			7,700
Operating lease liabilities		1,200			700
Other		2,100			1,200
Total deferred tax assets		278,600			258,500
Less: valuation allowance		(277,200	)		(257,900	)
Net deferred tax assets		1,400			600

Operating leases, right-of-use assets		(1,400	)		(600	)
Total deferred tax liabilities		(1,400	)		(600	)
Total net deferred tax assets	$	—		$	—

We record net deferred tax assets to the extent we believe these assets will more likely than not be realized. In making such determination, we consider all available positive and negative evidence, including scheduled reversals of deferred tax liabilities, projected future taxable income, tax planning strategies and recent financial performance. Forming a conclusion that a valuation allowance is not required is difficult when there is negative evidence such as cumulative losses in recent years. Because of our history of losses, the net deferred tax assets have been fully offset by a valuation allowance. The valuation allowance increased by $19,300,000 and $20,800,000 for the years ended December 31, 2020 and 2019 respectively.

As of December 31, 2020, we had domestic federal net operating loss carryforwards of approximately $898,100,000. Of this, $744,800,000 will expire at various dates beginning in 2021 through 2037 and the remaining will carryforward indefinitely under the new tax laws, but is subject to an 80% taxable income limitation for tax years beginning after 2021. As of December 31, 2020, we had state net operating loss carryforwards of approximately $408,000,000 expiring at various dates beginning in 2028 through 2040, if not utilized. We also had federal tax credit carryforwards of approximately $44,800,000 expiring at various dates beginning in 2021 through 2040, if not utilized. Our state tax credit carryforwards of approximately $20,100,000 carry forward indefinitely.

Utilization of net operating loss and tax credit carryforwards may be subject to an annual limitation due to ownership change limitations provided by the Internal Revenue Code and similar state provisions. Annual limitations may result in expiration of net operating loss and tax credit carryforwards before some or all of such amounts have

been utilized. The impact of any limitations that may be imposed due to such ownership changes has not yet been determined.

In March and December 2020, in response to the COVID-19 pandemic, the Coronavirus Aid, Relief and Economic Security Act, or the CARES Act, and the Consolidated Appropriations Act, 2021 were passed into law and provide additional economic stimulus to address the impact of the COVID-19 pandemic, including among other items, several U.S. income tax provisions related to, among other things, net operating loss carrybacks, alternative minimum tax credits, modifications to interest expense limitations, and an option to defer payroll tax payments for a limited period. We do not expect any significant benefit to our income tax provision as a result of this legislation.

We adopted the provision of the standard for accounting for uncertainties in income taxes on January 1, 2007. Upon adoption, we recognized no material adjustment in the liability for unrecognized tax benefits. At December 31, 2020, we had approximately $19,100,000 of unrecognized tax benefits, none of which would currently affect our effective tax rate if recognized due to our net deferred tax assets being fully offset by a valuation allowance.

A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows (in thousands):

If applicable, we would classify interest and penalties related to uncertain tax positions in income tax expense. Through December 31, 2020, there has been no interest expense or penalties related to unrecognized tax benefits.

We do not currently expect any significant changes to unrecognized tax benefits during the fiscal year ended December 31, 2021. In certain cases, our uncertain tax positions are related to tax years that remain subject to examination by the relevant tax authorities. Tax years for which we have carryforward net operating loss and credit attributes remain subject to examination by federal and most state tax authorities.