AQP4 and neuromyelitis optica: Robust AQP4 expression on the basolateral surface of choroid plexus epithelial cells in normal tissue coupled to the presence of immunoglobulin immunoreactivity and the nearly complete loss of AQP4 expression in these cells in NMO tissue indicates that the blood–CSF barrier may be a primary site for entry of NMO IgG into the CNS.