The FDA halted clinical trials of two of RegenxBio's gene therapies, RGX-111 and RGX-121, for rare childhood disorders.[1] The decision followed the discovery of a brain tumor in one participant in the RGX-111 study.[1] The tumor was found on a routine MRI in a five-year-old child who had received therapy about four years earlier and was symptom-free.[1] Initial genetic testing of the resected tumor revealed an adeno-associated virus (AAV) vector integration event associated with overexpression of the PLAG1 proto-oncogene.[1] The company has not confirmed a direct causal link with the treatment.[1] Nine other patients treated with RGX-111 and 32 patients with RGX-121 did not have tumors.[1] The suspension relates to a Phase I/II study of RGX-111 and a Phase I/II/III study of RGX-121.[1] CEO Curran Simpson emphasized that patient safety is a top priority.[1]