Regenxbio said the FDA placed a clinical hold on its Hunter syndrome gene therapy RGX-121 after investigators found masses on spines of five patients enrolled in the Campsiite study. The company reported the patients were asymptomatic, but the finding triggered regulatory action. Regenxbio said it now does not expect to file for FDA approval in the near future after the second clinical hold this year, following a prior safety concern linked to spine masses in an MPS type I program. The stop-and-review sequence reinforces heightened oversight for AAV-based therapies using CNS targets. The updated hold status complicates development timelines and may force protocol changes, additional imaging and monitoring, and further risk mitigation steps. For the gene therapy field, it continues to raise questions about long-term effects and imaging-based safety signals. Clinicians and researchers will wait for FDA dialogue and subsequent trial updates before considering future enrollments or regulatory steps.