AQP4 and neuromyelitis optica: However, NMO was traditionally considered as a variant of Multiple Sclerosis (MS), but after the discovery of a disease-specific immunoglobulin G antibody (NMO-IgG) that selectively binds to aquaporin-4 (AQP4; the most abundant water channel in the CNS), it is now considered as a distinct disease [2,3].