The U.S. Food and Drug Administration placed clinical holds on two Regenxbio gene‑therapy studies after a pediatric trial participant was diagnosed with a brain tumor. Regenxbio had been testing AAV‑based therapies for mucopolysaccharidosis (Hurler and Hunter syndromes); one program was weeks away from an FDA decision. The agency’s action immediately forced trial suspensions and sent Regenxbio shares sharply lower. Company statements and regulator notices confirm the holds are safety‑driven and will require additional investigation and follow‑up data. Regenxbio and external advisers will need to supply the FDA with data to evaluate causality, biodistribution, and tumor linkage before enrollment can resume. The pause underscores intensified regulatory scrutiny of in vivo AAV programs and the data requirements sponsors must meet for rare‑disease approvals.