AQP4 and multiple sclerosis: In the majority of patients, NMOSD is associated with the presence of autoantibodies against aquaporin-4 (AQP4) expressed on astrocytes, while multiple sclerosis is considered to be driven by cell-mediated autoreactivity against myelin peptides.1 Due to overlapping symptomatology, the differential diagnosis may be challenging but is crucial given distinct treatment and prognosis in both diseases.