In accordance with that hypothesis, numerous patients previously diagnosed as having variants of MS or ‘opticospinal MS’ based on the current clinicoradiological consensus criteria were found over the past decade to have newly discovered humorally mediated diseases, pathophysiologically distinct from MS, that are now termed AQP4-IgG-positive ‘neuromyelitis optica spectrum’ disorder (NMOSD) [28–33] and MOG-IgG-positive encephalomyelitis (EM) [17, 34–42]. Here, AQP4 is linked to myeloid sarcoma.