AQP4 and neuromyelitis optica: Initially considered a subtype of MS, a distinction was made 20 years ago due to the discovery that anti-AQP4 immunoglobulin G (IgG) antibodies are consistently present in the sera of the majority (~70–80%) of NMO cases during both active and remission phases of disease (Lennon et al., 2005; Sato et al., 2014).