AQP4 and neuromyelitis optica: While NMO had previously been regarded as variant of multiple sclerosis (MS), the recent detection of a highly specific serum biomarker for NMO, antibodies to the most abundant astrocytic CNS water channel aquaporin-4 (AQP4), as well as histopathological data has made clear that NMO is a condition distinct from MS [1], [3]–[7].