The U.S. Food and Drug Administration placed clinical holds on multiple Regenxbio gene‑therapy studies after the detection of a brain tumor in a pediatric trial participant. Regenxbio told investors the regulator paused trials of its AAV‑based programs while it investigates possible vector integration and oncogenic risk. The company’s stock plunged and timelines for pending approvals were disrupted as the agency demanded additional safety follow‑up and data reviews. The move affects ongoing studies for therapies including candidates for mucopolysaccharidoses and other rare genetic disorders, delaying development paths and regulatory decisions. FDA statements and Regenxbio filings indicate the agency flagged a safety signal that warrants suspension of enrollment and dosing while the events are adjudicated. The agency’s action spotlights persistent safety questions for in vivo AAV gene delivery and will likely trigger broader industry reassessments of monitoring, vector design and long‑term tumor surveillance.