MOG and myeloid sarcoma: Initially thought to be a rare and severe variant of multiple sclerosis (MS), it has become apparent that NMOSD is a pathologically distinct entity associated with serum antibodies to the astrocyte water channel aquaporin-4 (AQP4) in the majority of patients.1, , –4 Likewise, the recent discovery of myelin oligodendrocyte glycoprotein antibodies (MOG-IgG) has led to the detection of MOG-IgG in a portion of AQP4-IgG seronegative NMOSD patients.