AQP4 and myeloid sarcoma: Recently, an antibody against myelin oligodendrocyte glycoprotein (MOG-ab) was detected in a subgroup of exclusively AQP4-ab seronegative NMOSD patients [38–44], recurrent idiopathic optic neuritis (RION) patients, and a few MS patients [45, 46], further complicating the disease spectrum.