AQP4 and myeloid sarcoma: Based on the fact that aquaporin-4 (AQP4)-IgG is usually absent in MOG-IgG-positive patients [3–10], that the histopathology of inflammatory CNS lesions differs between MOG-IgG- and AQP4-IgG-positive patients [11–13], and that MOG-IgG are pathogenic both in vitro and in vivo [2, 14], MOG-IgG-related autoimmunity is now considered by many a disease entity in its own right, distinct both from classical MS and from AQP4-IgG-mediated neuromyelitis optica spectrum disorders (NMOSD) [15, 16].