Thus pointing out the possibility of a common autoimmune origin for both diseases, or the aforementioned worsening effect of the AQP4 autoantibody in patients with pre-existing immune diseases; previously unrecognized because of the lack of knowledge about the NMO IgG auto-AQP4 antibody as a diagnostic tool for NMO. This evidence concerns the gene AQP4 and neuromyelitis optica.