As of December 31, 2021 and 2020, our deferred tax assets (“DTAs”) were approximately $48.5 million and $45.8 million, respectively. The components of our deferred tax assets are summarized in the following table.

Current accounting standards require a valuation allowance against DTAs if, based on the weight of available evidence, it is more likely than not that some or all of the DTAs may not be realized. Due to the uncertainty surrounding the realization of these DTAs in future tax returns, all of the DTAs have been fully offset by a valuation allowance as of December 31, 2021 and 2020.

In assessing the realizability of DTAs, management considers whether it is more likely than not that some portion or all of the DTAs will not be realized. The ultimate realization of DTAs is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management considers the scheduled reversal of deferred tax liabilities (including the impact of available carryback and carryforward periods) and projected future taxable income in making this assessment. Based upon the level of historical taxable income and projections for future taxable income over the periods in which the DTAs are deductible, management believes it is more likely than not that the Company will not realize the benefits of these deductible differences or tax carryforwards as of December 31, 2021.

The Tax Cuts and Jobs Act (the “Tax Act”) was signed into law on December 22, 2017. The Tax Act reduced the U.S. federal corporate tax rate from 35% to 21%, effective January 1, 2018. Consequently, we recorded a decrease related to DTAs of $26.4 million with a corresponding net adjustment to a valuation allowance of $26.4 million for the year ended December 31, 2018. The Tax Act repealed the AMT for corporations, and permited any existing AMT credit carryforwards to be used to reduce the regular tax obligation in 2018, 2019 and 2020. Companies may continue using AMT credits to offset any regular income tax liability in years 2018 through 2020, with 50% of remaining AMT credits refunded in each of the 2018, 2019 and 2020 tax years, and all remaining credits refunded in tax year 2021. This results in full realization of an existing AMT credit carryforward irrespective of future taxable income. The CARES Act was signed into law on March 27, 2020 and allowed an acceleration of the full remaining AMT credit carryforward as a refund in the current year. Accordingly, the Company filed for and received the refund of all $621,000 of AMT credit carryforwards, plus interest, in June 2020.

As of December 31, 2021 and 2020, we had U.S. net operating loss carryforwards of approximately $164.1 million and $153.3 million, respectively. Of those amounts, $14.9 million relates to stock-based compensation tax deductions in excess of book compensation expense (“APIC NOLs”) as of December 31, 2021, that will be credited to additional paid-in capital when such deductions reduce taxes payable as determined on a "with-and-without" basis. Accordingly, these APIC NOLs will reduce federal taxes payable if realized in future periods, but NOLs related to such benefits are not included in the table above. As of December 31, 2017, we adopted ASU 2016-09 and thereby eliminated all APIC NOLs with a full offset to a valuation allowance.

As of December 31, 2021 and 2020, we also had state net operating loss carryforwards of approximately $20.1 million. The state net operating loss carryforwards will begin expiring in 2032.

As of December 31, 2021 and 2020, we had U.S. R&D credit carryforwards of approximately $9.1 million and $8.9 million, respectively.

There were no expirations of U.S. NOL carryforwards during 2021 or 2020. U.S. R&D credit carryforwards of $0 and $72,000 expired during 2021 and 2020, respectively.

The details of our U.S. net operating loss and federal R&D credit carryforward amounts and expiration dates are summarized in the following table.

Under Sections 382 and 383 of the Internal Revenue Code (“IRC”) of 1986, as amended, the utilization of U.S. net operating loss and R&D tax credit carryforwards may be limited under the change in stock ownership rules of the IRC. The Company completed a Section 382 analysis through December 31, 2021 and believes that a Section 382 ownership change has not occurred.

Reconciliations between the statutory federal income tax rate and our effective tax rate for continuing operations are presented in the following table.