While target and pathogenic relevance of this humoral response is still under debate [2], autoantibodies against aquaporin-4 (AQP-4) allow to distinguish NMO from other central nervous system (CNS) demyelinating conditions, promote development of NMO-like lesions in animal models [3] and may correlate with progression of NMO itself [4]. Here, AQP4 is linked to neuromyelitis optica.