A substantial portion of patients with another form of ON, frequently in a form of unilateral or bilateral papillitis, either isolated or associated with myelitis in adults, which phenotypically resembles Devic’s disease, but is AQP4-Ig negative—may instead be myelin oligodendrocyte glycoprotein (MOG) IgG positive [15]. The gene discussed is MOG; the disease is neuromyelitis optica.