The concept of neuromyelitis optica spectrum disorder (NMOSD) is changing, with a disease spectrum emerging that includes aquaporin 4 (AQP4) IgG-seropositive NMOSD, myelin oligodendrocyte glycoprotein (MOG) antibody-associated disease (MOGAD), and double-seronegative NMOSD. The gene discussed is AQP4; the disease is neuromyelitis optica.