The U.S. Food and Drug Administration has placed two Regenxbio gene‑therapy trials on clinical hold after the identification of a brain tumor in a pediatric trial participant. Regenxbio’s programs involved AAV-based therapies for mucopolysaccharidosis type I and II (Hurler and Hunter syndromes); the regulator paused studies while it investigates potential vector-related integration and safety signals. Regenxbio disclosed the hold following internal reviews and regulatory queries; company statements and media reports indicate the decision affects both an IND nearing regulatory decision and an earlier‑phase program. The agency’s action triggers safety reviews, extended follow‑up of treated patients, and likely protocol amendments for ongoing AAV studies across the field. The suspension underscores renewed FDA scrutiny of AAV integration risk and tumorigenicity after long‑term follow‑up findings. Developers and manufacturers of systemic and CNS‑directed AAV therapies should expect intensified data requests, longer monitoring expectations, and possible adjustments to preclinical genotoxicity packages.