AQP4 and neuromyelitis optica: Neuromyelitis optica spectrum disorders (NMOSD) are severe inflammatory diseases of the central nervous system (CNS), with the presence of aquaporin 4 (AQP4)-specific serum antibodies (AQP4-abs) in the vast majority of patients [1,2], and with the presence of myelin oligodendrocyte glycoprotein (MOG)-specific antibodies (MOG-abs) in approximately 40% of all AQP4-absnegative NMOSD patients [3,4].