AQP4 and myeloid sarcoma: Myelin oligodendrocyte glycoprotein antibody-associated encephalomyelitis (MOG-EM; also termed MOG antibody-associated disease, MOGAD) has recently been established to be one of the most important differential diagnoses of multiple sclerosis (MS), aquaporin-4 (AQP4) immunoglobulin (Ig) G-seropositive neuromyelitis optica spectrum disorders (NMOSD), and a number of other inflammatory disorders of the central nervous system (CNS) [10, 12, 20, 23].