The U.S. Food and Drug Administration placed clinical holds on Regenxbio’s gene‑therapy programs after investigators detected a brain neoplasm in a pediatric trial participant. Regenxbio said the pause affects studies of therapies for mucopolysaccharidosis (MPS) I and II; the action follows preliminary analysis linking AAV integration to proto‑oncogene overexpression. Regulators moved swiftly to pause patient enrollment and require additional safety data. Company filings and agency notices indicate the hold will delay timelines for a therapy that had been nearing a regulatory decision. Regenxbio has pledged cooperation with the FDA and launched internal reviews; the market reaction was immediate, with share prices tumbling on the safety signal. The cases highlight persistent integration and genotoxicity concerns for AAV vectors and will likely prompt re‑examination of monitoring protocols across the gene‑therapy field. Researchers and sponsors developing AAV programs now face heightened scrutiny on long‑term vector biodistribution and insertional events; independent data monitoring committees and regulators are expected to demand deeper molecular analyses before trials can resume. The situation illustrates how a single adverse oncologic finding can trigger broad regulatory reassessment across rare‑disease gene therapy portfolios.