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Income Taxes | 12 Months Ended
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Dec. 31, 2023
Income Tax Disclosure [Abstract] |
Income Taxes | Income Taxes The Company’s operations and income tax components are solely in the United States. From inception through 2023, the Company has only generated pretax losses in the United States and has not generated any pretax income or loss outside of the United States. The Company did not record a provision (benefit) for income taxes for the years ended December 31, 2023 and 2022. The Company accounts for income taxes in accordance with ASC 740, which requires that the tax benefit of net operating losses, temporary differences and credit carryforwards be recorded as an asset to the extent that management assesses that realization is "more likely than not." Realization of the future tax benefits is dependent on the Company's ability to generate sufficient taxable income within the carryforward period. Because of the Company's recent history of operating losses, management believes that recognition of the deferred tax assets arising from the above-mentioned future tax benefits is currently not likely to be realized and, accordingly, has provided a full valuation allowance. A reconciliation of the statutory U.S. federal rate to the Company’s effective tax rate is as follows:
| December 31,
| 2023 |  | 2022
Tax at federal statutory rate | 21.0 | % |  | 21.0 | %
State taxes, net of federal benefit | 4.5 | % |  | 7.2 | %
Research and development tax credit | 1.8 | % |  | 1.4 | %
Stock-based compensation | (0.8) | % |  | (1.9) | %
Nondeductible interest expense | (0.5) | % |  | (0.5) | %
FIN 48 reserve | (0.3) | % |  | (0.2) | %
Change in valuation allowance | (23.3) | % |  | (26.8) | %
Other | (2.4) | % |  | (0.2) | %
Total | — | % |  | — | %
The tax effects of temporary differences that give rise to significant components of the deferred tax assets and liabilities are as follows (in thousands):
| December 31,
| 2023 |  | 2022
Deferred tax assets: |  |  |
Net operating loss carryforwards | $ | 41,864 |  |  | $ | 40,884 |
Research and development credits | 13,069 |  |  | 12,088 |
Research and development expenditures, capitalized for tax | 8,271 |  |  | 4,563 |
Fixed assets and inventory | 151 |  |  | 159 |
Accruals and reserves | 1,633 |  |  | 1,180 |
Interest expense carryforward | 5,033 |  |  | 3,850 |
Operating lease liability | 3,958 |  |  | 4,442 |
Other | 1,718 |  |  | 1,521 |
Gross deferred tax assets | 75,697 |  |  | 68,687 |
Deferred tax liabilities: |  |  |
Operating lease right-of-use asset | (3,436) |  |  | (3,910) |
Total deferred tax liabilities | (3,436) |  |  | (3,910) |
Valuation allowance | (72,261) |  |  | (64,777) |
Net deferred tax assets | $ | — |  |  | $ | — |
Beginning January 1, 2022, the Tax Cuts and Jobs Act, or the Tax Act, eliminated the option to deduct research and development expenditures in the current year and requires taxpayers to capitalize such expenses pursuant to Internal Revenue Code Section 174. The capitalized expenses are amortized over a 5-year period for domestic expenses and a 15-year period for foreign expenses. As a result of this provision of the Tax Act, deferred tax assets related to capitalized research expenses increased by $8.3 million. We recognize deferred income taxes for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes, as well as for tax attribute carryforwards. We regularly evaluate the positive and negative evidence in determining the realizability of our deferred tax assets. Based upon the weight of available evidence, which includes our historical operating performance and reported cumulative net losses since inception, we maintained a full valuation allowance on the net deferred tax assets as of December 31, 2023 and 2022. We intend to maintain a full valuation allowance on our deferred tax assets until sufficient positive evidence exists to support reversal of the valuation allowance. The valuation allowance increased by $7.5 million and $12.8 million during the years ended December 31, 2023 and 2022, respectively. The current year change in the valuation allowance is primarily the result of $7.0 million increase in gross deferred tax assets net of a $0.5 million decrease in deferred tax liabilities. As of December 31, 2023, the Company had net operating loss, or NOL, carryforwards of $155.0 million and $150.3 million available to reduce future taxable income, if any, for federal and state income tax purposes, respectively. The federal and state NOL carryforwards begin expiring in 2023 and 2028, for federal and state purposes, respectively. As of December 31, 2023, the amount of federal NOL carryforwards that does not expire is $103.3 million (subject to certain utilization limitations). As of December 31, 2023, the Company had research and development credit carryforwards of $4.1 million and $13.2 million available to reduce future taxable income, if any, for federal and state income tax purposes, respectively. The federal credit carryforwards begin expiring in 2036 and the state credits carryforward indefinitely. Utilization of the Company’s NOL and tax credit carryforwards may be subject to a substantial annual limitation due to the ownership change provisions included in the Internal Revenue Code, or Section 382, and similar state provisions. An annual limitation may result in the expiration of NOL and credit carryforwards before utilization. The Company conducted Section 382 studies as of 2016 and 2021 and has determined that it experienced Section 382 ownership changes in 2016 and in 2021. The 2016 ownership change resulted in permanent limitations of its NOL and credit carryforwards. The 2021 ownership change did not result in permanent limitations of its NOL or credit carryforwards. It has been determined that $226.8 million and $150.7 million of federal and state NOL carryforwards, respectively, have been permanently limited and will expire unutilized. It has also been determined that $10.4 million of federal research and development credits have been permanently limited and will expire unutilized. The gross deferred tax assets disclosed above exclude NOL and credit carryforwards that are expected to expire due to the Section 382 limitation. The American Rescue Plan Act of 2021 was signed into law on March 11, 2021 expanding the definition of covered employees as defined under Section 162(m). The provisions under the expanded definition of covered employees did not have a material impact on the Company’s tax positions for the year ended December 31, 2022 or 2023. California Assembly Bill 85 (AB 85) was signed into law by Governor Gavin Newsom on June 29, 2020. The legislation suspends the California NOL deductions for 2020, 2021, and 2022 for certain taxpayers and imposes a limitation on certain California tax credits for 2020, 2021, and 2022. The legislation disallows the use of California NOL deductions if the taxpayer recognizes business income and its adjusted gross income is greater than $1,000,000. The carryover periods for NOL 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 the Company’s taxable loss position, this legislation did not impact the tax provision for the years ended December 31, 2022 or 2023. California Senate Bill 113 (SB 113), was signed into law by Governor Newsom on February 9, 2022. The legislation contains important California tax law changes, including reinstatement of business tax credits and net NOL deductions limited by AB 85 mentioned above. The new tax law should be accounted for under ASC 740 in the period of enactment (2022) but is not expected to have a material impact on the Company’s tax provision due to its taxable loss position. On August 16, 2022, the President signed into law H.R. 5376 (commonly called the “Inflation Reduction Act of 2022”). The primary tax provisions in the new law include an alternative minimum tax on certain large corporations, a tax on stock buybacks and certain energy-related tax credits, each of which become effective after December 31, 2022. The provisions of the Inflation Reduction Act of 2022 does not have a material impact on the Company’s financial statements and related disclosures. As of December 31, 2023, the Company had unrecognized tax benefits of $1.7 million related to $4.1 million and $13.2 million of federal and state research and development tax credit carryforwards, respectively. The unrecognized tax benefits, if recognized, would not have an impact on the Company's effective tax rate, due to the valuation allowance. It is unlikely that the amount of unrecognized tax benefits will significantly change over the next twelve months. No liability related to uncertain tax positions is recorded in the financial statements. A reconciliation of the beginning and ending unrecognized tax benefit amount is as follows (in thousands):
| Year Ended December 31,
| 2023 |  | 2022
Beginning balance | $ | 1,601 |  |  | $ | 1,489 |
Increase in balance related to tax positions taken during the current year | 136 |  |  | 112 |
Decrease in balance related to tax positions taken during prior years | (7) |  |  | — |
Ending balance | $ | 1,730 |  |  | $ | 1,601 |
It is the Company’s policy to include penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary. The Company determined that no accrual for interest and penalties related to unrecognized tax benefits was required as of December 31, 2023 and December 31, 2022. All of the Company’s tax years will remain open for examination by the federal and state authorities for 3 and 4 years, respectively, from the date of utilization of its tax attributes.

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