REGENXBIO Inc. Income Taxes Disclosure

13. Income Taxes

Due to taxable losses and a full valuation allowance against its deferred tax assets, the Company did not record a provision for income taxes in the U.S. (federal or state) or any foreign jurisdictions for the years ended December 31, 2025 and 2024.

Beginning in 2022, the Tax Cuts and Jobs Act of 2017 (the TCJA) eliminated the option to deduct research and development expenses currently and required taxpayers to amortize such costs over a period of five years for expenses incurred in the United States and a period of 15 years for expenses incurred outside the United States. This provision of the TCJA resulted in deferred tax assets of $122.1 million as of December 31, 2024 related to capitalized research and development expenses, net of amounts amortized to date.

The One Big Beautiful Bill Act (OBBBA) was enacted in July 2025. The OBBBA amended U.S. federal tax laws by restoring the option for immediate expense recognition of research and development expenses incurred in the United States and making permanent the ability to claim 100% bonus depreciation on qualified property, among other changes. The Company currently expects to elect these options and deduct U.S. research and development expenses incurred in the current period, along with 100% bonus depreciation on eligible property placed in service, beginning in 2025. Research and development expenses incurred outside the United States will continue to be capitalized and amortized over a period of 15 years for tax purposes. Unamortized U.S. research and development expenses incurred prior to 2025 may be amortized over their remaining life, or over a period of one or two years beginning in 2025. The Company currently expects to amortize these amounts over two years beginning in 2025. The enactment of the OBBBA resulted in a decrease in deferred tax assets for capitalized research and development expenses and an increase in deferred tax assets for net operating loss (NOL) carryforwards during the year ended December 31, 2025. However, due to taxable losses and a full valuation allowance against its deferred tax assets, the enactment of the OBBBA had no material impact to the Company's income tax provision for the year ended December 31, 2025. The Company continues to evaluate the impact the OBBBA will have on its consolidated financial statements in future periods.

The Organization for Economic Co-operation and Development (OECD) has introduced BEPS Pillar 2 rules that impose a global minimum tax rate of 15%. Numerous countries enacted corresponding legislation that is effective at various dates beginning as early as January 1, 2024. These rules generally apply to multinational companies with consolidated revenue of at least €750.0 million in at least two of the four preceding fiscal years. Based on the revenue thresholds established in the BEPS Pillar 2 rules, these changes do not have an impact on the Company’s income tax provision for the years ended December 31, 2025 and 2024. Further, under administrative guidance issued in January 2026, even if the Company were to exceed the applicable revenue threshold in future periods, it would be eligible to elect the Side-by-Side Safe Harbor, which is expected to mitigate the impact of the Pillar 2 rules on the Company’s income tax provision.

The Company evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets as of December 31, 2025 and 2024. Based on the Company’s history of operating losses, and other relevant facts and circumstances, the Company concluded that it was more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company provided a full valuation allowance for its deferred tax assets as of December 31, 2025 and 2024. The valuation allowance increased by $36.1 million and $57.9 million during the years ended December 31, 2025 and 2024, respectively. The increase in the valuation allowance during the year ended December 31, 2025 was due primarily to federal and state NOLs and research and development tax credits generated during the period, partially offset by a decrease in capitalized research and development expenses as a result of the enactment of the OBBBA. The increase in the valuation allowance during the year ended December 31, 2024 was

due primarily to an increase in capitalized research and development expenses, and federal and state NOLs and research and development tax credits generated during the period.

As of December 31, 2025, the Company had U.S. federal and state research and development tax credit carryforwards of approximately $95.1 million which may be available to reduce future income tax liabilities. The calculation of these credits requires assumptions to be made by the Company to estimate qualified research expenses. The Company conducts formal studies to document the qualified activities and expenses used to calculate these credits however a portion of these credits may be subject to future examinations which have not yet occurred, the results of which may result in an adjustment to the Company’s credit carryforwards. The Company accounts for uncertain tax positions in accordance with the requirements of ASC 740, and recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. Further, a full valuation allowance has been provided against the net credit carryforwards and, if an adjustment is required upon the completion of the study, this adjustment would be offset by an adjustment to the deferred tax asset established for the research and development credit carryforwards and the valuation allowance. As of December 31, 2024, the Company had total unrecognized tax benefits of $0.1 million which were reserved against its research and development tax credit carryforwards as uncertain tax positions. The Company released all of its reserves for unrecognized tax benefits during the year ended December 31, 2025 due to the expiration of the associated statute of limitations, and had no unrecognized tax benefits as of December 31, 2025.

Under the provisions of the Internal Revenue Code, the Company’s NOL and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. NOL and tax credit carryforwards may be subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several financings since its inception which may have resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code, or could result in a change in control in the future.

The Company and its subsidiaries file income tax returns in the United States, at the federal level and in various states, and in foreign jurisdictions. The U.S. federal and state income tax returns are generally subject to tax examinations for the tax years ended December 31, 2021 onward. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service or state tax authorities to the extent utilized in a future period.