U.S. regulators put Regenxbio’s clinical programs on hold after detection of a central nervous system neoplasm in a pediatric participant, prompting an FDA clinical hold across at least two AAV‑based trials. Company disclosures and Fierce Biotech reporting state the agency suspended studies for RGX‑121 (MPS I) and RGX‑111 (MPS II) pending investigation of the adverse event and its potential link to vector integration. Regenxbio said the pause affects active studies and could delay near‑term regulatory timelines. The hold underscores heightened FDA scrutiny of AAV integration and long‑term genotoxicity signals, and companies developing AAV therapeutics now face increased emphasis on monitoring, biodistribution, and safety‑signal adjudication during early‑phase testing.