TScan Therapeutics, Inc. Income Taxes Disclosure

8. Income Taxes

During the years ended December 31, 2025 and 2024, the Company did not record an income tax provision due to the losses incurred and a full valuation allowance provided on the net deferred tax assets.

In December 2023, the FASB issued ASU 2023-09, "Income Taxes (Topic 740): Improvements to Income Tax Disclosures,” effective for annual periods beginning after December 15, 2024. ASU 2023-09 requires companies to present a detailed reconciliation of the statutory and effective income tax rates, including specified categories such as state and local taxes, foreign taxes, tax credits, and changes in valuation allowances, to provide greater transparency into the factors affecting the effective tax rate. The standard also mandates disclosure of income taxes paid, disaggregated by federal, state, and foreign jurisdictions, enabling users to better understand the company’s cash tax payments across different tax authorities. Furthermore, companies must describe significant tax positions and valuation allowances, including the nature and amounts of such positions, and the judgments or assumptions underlying their recognition or measurement. ASU 2023-09 permits companies to apply these enhanced disclosure requirements either retrospectively to all periods presented or prospectively to periods beginning after the adoption date; the Company has elected to adopt the standard prospectively. The adoption of ASU 2023-09 had no impact to the Company’s consolidated balance sheets, consolidated statements of comprehensive (loss) income, or consolidated statements of cash flows, as ASU 2023-09 affects disclosures only.

On July 4, 2025, the One Big Beautiful Bill Act (the "OBBBA") was enacted, providing taxpayers the option to fully deduct or continue capitalizing and amortizing domestic R&D expenditures under new Code Section 174A, effective for tax years beginning after December 31, 2024. The OBBBA also provides certain eligible taxpayers with the option to accelerate and deduct the remaining unamortized domestic R&D costs incurred during taxable years ending after December 31, 2021 and before January 1, 2025. The Company intends to continue amortizing domestic R&D costs incurred during taxable years ending after December 31, 2021 and before

January 1, 2025. As of December 31, 2025, $129.3 million remains unamortized related to domestic R&D costs. Final elections will be made with the 2025 tax return filing.

Deferred tax assets and liabilities reflect the net tax effects of net operating loss carryovers and temporary differences between the carrying amount of assets and liabilities for financial reporting and the amounts used for income tax purposes. Significant components of the Company’s deferred tax assets and liabilities were as follows (in thousands):

In determining the need for a valuation allowance, the Company has given consideration to its cumulative book income and loss positions. The Company has assessed the available means of recovering deferred tax assets, including the ability to carryback net operating losses, the existence of reversing taxable temporary differences, the availability of tax planning strategies and forecasted future taxable income. As of December 31, 2025, the Company maintains a full valuation allowance against its net deferred tax assets. The valuation allowance increased by $39.7 million and $39.3 million during the years ended December 31, 2025 and 2024, respectively.

As of December 31, 2025, the Company had U.S. federal net operating loss carryforwards of approximately $174.7 million. The U.S. federal net operating losses have an indefinite life carryforward. As of December 31, 2025, the Company had Massachusetts net operating loss carryforwards of approximately $186.6 million that expire at various dates through 2045. As of December 31, 2025, the Company had U.S. R&D federal credit carryforwards of approximately $21.8 million that expire at various dates through 2045. As of December 31, 2025, the Company had U.S. state R&D tax credit carryforwards of approximately $13.0 million that expire at various dates through 2040.

Under Sections 382 and 383 of the U.S. Internal Revenue Code, if a corporation undergoes an ownership change, the corporation's ability to use its pre-change net operating loss carryforwards and other pre-change attributes, such as net operating losses and research tax credits, to offset its post-change income and taxes may be limited. In general, an ownership change generally occurs if there is a cumulative change in ownership by 5% stockholders that exceeds 50 percentage points over a rolling three-year period. Similar rules may apply under U.S. state tax laws. The Company has experienced an ownership change in the past and may experience ownership changes in the future as a result of future transactions in its share capital, some of which may be outside the control of the Company. As a result, if the Company earns net taxable income, its ability to use its pre-change net operating loss carryforwards, or other pre-change tax attributes, to offset U.S. federal and state taxable income and taxes is subject to significant limitations.

The Company accounted for uncertain tax positions using a more likely than not threshold for recognizing and resolving uncertain tax positions. The evaluation of uncertain tax positions is based on factors that include, but are not limited to, changes in tax law, the measurement of tax positions taken or expected to be taken in tax returns, the effective settlement of matters subject to audit, new audit activity and changes in facts or circumstances related to a tax position. The Company evaluates uncertain tax positions on an annual basis and adjusts the level of the liability to reflect any subsequent changes in the relevant facts surrounding the uncertain positions. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. For the years ended December 31, 2025 and 2024, there were no accrued interest or penalties in the consolidated statements of operations.

The Company is subject to taxation for federal and Massachusetts purposes. As of December 31, 2025, the Company is subject to examination by these taxing authorities for all years since inception in 2018.