AQP4 and neuromyelitis optica: Neuromyelitis optica (NMO) is a rare, disabling, sometimes fatal, central nervous system (CNS) demyelinating disease characterized by severe attacks of optic neuritis and transverse myelitis.1 NMO is considered to be primarily a humoral autoimmune disease, as a majority of NMO patients develop autoantibodies (NMO immunoglobulin [Ig]G) against aquaporin 4 (AQP4),2 the predominant CNS water channel, which is abundantly expressed on astrocytes.