The U.S. Food and Drug Administration placed clinical holds on Regenxbio’s gene therapy programs after investigators identified a brain tumor in a treated participant. The hold affects trials for therapies targeting mucopolysaccharidoses, including a program that was approaching an FDA decision. Regenxbio disclosed the safety finding and regulators paused dosing to investigate potential vector integration and oncogenic signals. Company filings and regulatory notices show the action arrived as Regenxbio prepared for key regulatory milestones. The hold triggered an immediate market reaction and will force Regenxbio and sponsors to provide additional safety data, long-term follow-up, and integration analyses requested by the agency. The decision underscores heightened FDA scrutiny of AAV-based gene therapies after rare but serious adverse events. Investigators and independent reviewers will need to determine whether the tumor is related to vector integration, transgene expression, or unrelated pathology. The outcome will influence ongoing development timelines for affected programs, potential label expectations, and risk-mitigation strategies across other gene therapy sponsors with similar constructs or delivery platforms.