Although AQP4 + NMOSD, as defined by the International Panel for Neuromyelitis Optica (NMO) Diagnosis (IPND) criteria [4], and MOG-IgG-seropositive patients present comparable clinical and radiological features, current data support the assumption that the presence of anti-MOG-IgG defines a disorder pathogenetically distinct from both NMOSD and MS [5–9], and which has been recently termed myelin oligodendrocyte glycoprotein (MOG)-associated encephalomyelitis (MOG-EM) [5] or MOG antibody-associated disease (MOGAD) [10]. The gene discussed is AQP4; the disease is neuromyelitis optica.