AQP4 is widely expressed in the body, including the kidneys, stomach and muscle, but its M23 isoform occurs in orthogonal arrays of particles in astrocytes, which has been proposed to be one of the reasons why astrocytes appear to be a prime target of complement-mediated lysis when anti-AQP4 antibodies (NMO-IgG) bind to these structures. This evidence concerns the gene AQP4 and neuromyelitis optica.