Neuromyelitis optica (NMO) was classified as a subtype of multiple sclerosis (MS) for decades2,3, but it could be distinguished from MS after the NMO-IgG autoantibody was discovered in 2004, which was later identified as directed against the aquaporin-4 protein (AQP4)4. The gene discussed is AQP4; the disease is myeloid sarcoma.