The FDA placed clinical holds on two Regenxbio gene therapy programs after a child in a trial developed a brain tumor, halting work on a treatment that was weeks from a regulatory decision. Regenxbio confirmed the pause and said regulators flagged a neoplasm in a participant; company filings and SEC notices followed. The holds affect RGX-111 and RGX-121 programs and have immediate commercial and regulatory consequences, forcing an internal safety review and triggering a steep market reaction. The action echoes broader agency scrutiny of AAV and in vivo gene therapies after rare integration or oncogenesis signals. Regenxbio said it will cooperate with the FDA and provide follow-up data; external analysts flagged the risk to near-term approval timelines and to investor confidence in related portfolios. For developers and investors, the event underscores the sensitivity of late-stage gene therapy programs to single serious adverse events and regulators’ willingness to impose rapid trial holds.