Aquaporin-4-immunoglobulin G-seropositive neuromyelitis optica spectrum disorder (AQP4-IgG+NMOSD) is a relapsing autoimmune astrocytopathy.1,2 Although treatment options for attack prevention in AQP4-IgG+NMOSD have rapidly expanded in recent years, biomarkers for monitoring disease activity are still insufficiently established and thus represent an important unmet need. Here, AQP4 is linked to neuromyelitis optica.