However, with the detection of a highly specific serum biomarker, an autoantibody targeting the most abundant astrocytic water channel aquaporin-4 (AQP4), in patients with NMO but not MS or other inflammatory or neurological diseases, it has become clear that NMO is a disease entity distinct from MS despite a considerable overlap in clinical presentation and paraclinical findings [138-148]. Here, AQP4 is linked to neuromyelitis optica.