Regenxbio said the FDA placed a second clinical hold on its Hunter syndrome gene therapy, RGX-121 (clemidsogene lanparvovec), after investigators found masses on spines of five patients in the Campsiite study. The company said the patients were asymptomatic, but the safety signal derailed plans for near-term regulatory resubmission. The development follows earlier concerns in related AAV gene therapy settings, where MRI-detected tumors were conclusively linked to treatment. Regenxbio has now indicated it does not expect to file for FDA approval in the near future after this second pause. The hold also increases pressure on the broader in vivo AAV gene therapy field to refine monitoring standards and define imaging triggers for intervention and patient management.