For a long time, NMOSD had been seen as a rare variant of MS; however, the seminal discovery of a highly specific serum IgG autoantibody to the astrocyte water channel aquaporin-4 (AQP4) in up to 80% of NMOSD patients and subsequent research into the role of these antibodies in disease pathogenesis and lesion formation has made clear that this is a condition distinct from MS (11–17). Here, AQP4 is linked to myeloid sarcoma.