Serum antibodies to myelin-oligodendrocyte glycoprotein (MOG) are biomarkers of MOG-IgG-associated disorder (MOGAD), a demyelinating disease distinct from both multiple sclerosis (MS) and aquaporin-4-IgG neuromyelitis optica spectrum disorder (NMOSD). The gene discussed is AQP4; the disease is neuromyelitis optica.