AQP4 and neuromyelitis optica: During the last 15 years, the global concept of inflammatory demyelinating disorders of the central nervous system (CNS) has radically changed with the identification of specific autoantibody-associated conditions distinct from multiple sclerosis (MS), namely aquaporin-4 (AQP4)-IgG-positive neuromyelitis optica spectrum disorder (AQP4-IgG+NMOSD) and myelin oligodendrocyte glycoprotein (MOG)-IgG-associated disease (MOGAD) (1–4).