AQP4 and neuromyelitis optica: In addition to that, a number of questions remain to be answered: why other organs which highly express AQP4 (e.g. kidneys, lung, inner ear and intestine) are not affected in the disease; why the lesions are predominantly located in the spinal cord and optic nerves, although AQP4 is ubiquitously expressed throughout the nervous system; what is the preceding event opening the BBB to allow the peripheral anti-AQP4 antibody enter the brain; why a NMO-like phenotype does not follow AQP4 deletion in mice.