The US Food and Drug Administration (FDA) has rejected Regenxbio's application for approval of the gene therapy RGX-121 for the treatment of the rare disease Hunter syndrome (MPS II).[3] The decision came on Monday, shortly before the scheduled date of February 8.[3] At the same time, the FDA suspended clinical trials of two experimental gene therapies RGX-111 and RGX-121 after the discovery of a brain tumor in one patient.[1][2][3][4] The patient, a five-year-old child, received RGX-111 four years ago and remains symptom-free.[2][3] Genetic analysis revealed integration of the AAV vector genome associated with overexpression of the PLAG1 proto-oncogene.[1][2] No additional tumor events were reported in 9 patients treated with RGX-111 or in 32 patients with RGX-121, including those dosed seven years ago.[2][3] Regenxbio is awaiting details from the FDA and expresses surprise at the suspension of RGX-121.[1][2][3] These therapies target the rare diseases Hurler syndrome (MPS I) and Hunter syndrome, which cause cognitive, cardiac and other disorders.[4]