FDA has placed another clinical hold on Regenxbio’s Hunter syndrome AAV gene therapy, RGX-121, after investigators found asymptomatic spine MRI masses in five patients enrolled in the Campsiite study. Regenxbio said the program’s next steps—including a previously anticipated refiling—are now uncertain. The action follows a prior clinical pause tied to a tumor signal observed in a related AAV spine MRI context. Regenxbio’s latest update underscores the regulatory sensitivity around imaging findings even when patients lack symptoms. The hold is operationally significant for the company’s timelines and financing, and it adds to broader gene therapy development scrutiny around long-term follow-up, imaging protocols, and causality assessment.