Neuromyelitis optica (NMO), a subtype of neuromyelitis optica spectrum disorder (NMOSD), is an autoimmune inflammatory disease characterized by severe attacks involving the optic nerves, causing neuritis, and the spinal cord, causing longitudinally extensive transverse myelitis (LETM), commonly associated with antibodies against aquaporin-4 (AQP4). The gene discussed is AQP4; the disease is neuromyelitis optica.