Neuromyelitis optica (NMO) is a chronic, rare, inflammatory CNS disorder primarily manifesting as optic neuritis and longitudinally extensive transverse myelitis lesions and characterized by the presence of pathogenic autoantibodies (immunoglobulin G [IgG]) against aquaporin-4 (AQP4-IgG). The gene discussed is AQP4; the disease is neuromyelitis optica.