In 2004, Lennon et al. discovered a specific autoantibody, NMO-IgG, in patient serum2, and it has subsequently been learned that the target antigen of NMO-IgG is aquaporin 4 (AQP4), the main water channel protein of the central nervous system that is densely expressed in astrocyte endfeet. This evidence concerns the gene AQP4 and neuromyelitis optica.