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,
	2019			2018
	(In thousands)
Net operating loss carryforwards	$	204,600		$	192,100
Research credits		38,400			35,500
Capitalized research and development		5,900			2,500
Stock-based compensation		7,700			6,400
Operating lease liabilities		700			—
Other		1,200			600
Total deferred tax assets		258,500			237,100
Less: valuation allowance		(257,900	)		(237,100	)
Net deferred tax assets		600			—

Operating leases, right-of-use assets		(600	)		—
Total deferred tax liabilities		(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 $20,800,000 and $4,300,000 for the years ended December 31, 2019 and 2018, respectively, and decreased by $89,400,000 during the year ended December 31, 2017. No income tax benefit was realized from stock options exercised in 2019 because our net deferred tax assets have been fully offset by a valuation allowance.

As of December 31, 2019, we had domestic federal net operating loss carryforwards of approximately $859,200,000. Of this, $769,500,000 will expire at various dates beginning in 2020 through 2037 and the remaining will carryforward indefinitely under the new tax laws, but is subject to an 80% taxable income limitation. As of December 31, 2019, we had state net operating loss carryforwards of approximately $346,500,000 expiring at various dates beginning in 2028 through 2039, if not utilized. We also had federal research and development tax credit carryforwards of approximately $39,400,000 expiring at various dates beginning in 2020 through 2039, if not utilized. Our state research and development tax credit carryforwards of approximately $19,600,000 carry forward indefinitely.

Due to the change of ownership provisions of the Tax Reform Act of 1986, utilization of a portion of our domestic net operating loss and tax credit carryforwards may be limited in future periods. Further, a portion of the carryforwards may expire before being applied to reduce future income tax liabilities.

On December 22, 2017, the Tax Cuts and Jobs Act of 2017, or 2017 Tax Act, was signed into law. Among other things, the 2017 Tax Act permanently lowers the corporate federal income tax rate to 21% from the previous maximum rate of 35%, effective for tax years including or commencing January 1, 2018. In accordance with GAAP, we remeasured the carrying value of our deferred tax assets as of December 31, 2017 using the new enacted corporate federal income tax rate of 21%. This remeasurement reduced our aggregate net deferred tax assets and correspondingly reduced the valuation allowance by approximately $102,300,000 in 2017. The remeasurement did not impact our financial statements.

In accordance with Staff Accounting Bulletin 118, as of December 31, 2017, we made a reasonable estimate of the effects of the 2017 Tax Act on our existing deferred tax assets. Our preliminary estimate and the remeasurement of our deferred tax assets was subject to further analysis related to certain matters, such as developing interpretations of the provisions of the 2017 Tax Act, changes to certain estimates and the filing of our tax returns. U.S. Treasury regulations, administrative interpretations or court decisions interpreting the 2017 Tax Act may require further adjustments and changes in our estimates. In the fourth quarter of 2018, we completed our analysis to determine the effect of the 2017 Tax Act. No material adjustments were noted from the completion of the analysis as of December 31, 2018.

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, 2019, we had approximately $17,700,000 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, 2019, 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, 2020. 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.