The antigen recognized for NMO-IgG is aquaporin-4 (AQP4), the most abundantly expressed aquaporin in the CNS[4-8], highly localized in astrocyte membranes facing blood vessel capillaries and in ependymal cells that line the cerebrospinal fluid-filled ventricles and layer of the meninges surrounding the brain and spinal cord[7]. Here, AQP4 is linked to neuromyelitis optica.