AQP4 and in situ carcinoma: ON can be the initial event in multiple sclerosis (MS), including clinically isolated syndrome (CIS) (3), in aquaporin-4-IgG positive (AQP4-IgG+) and seronegative neuromyelitis optica spectrum disorders (NMOSD) (7), and in myelin oligodendrocyte glycoprotein-IgG (MOG-IgG+)-associated disease (MOGAD) (8).