AQP4 and neuromyelitis optica: NMO is characterised by a longitudinally extensive spinal cord lesion spanning three or more vertebral segments, the lack of symptomatic brain lesions [3]–[5] and the presence of antibodies targeting the aquaporin-4 water channel (AQP4) in a significant proportion of patients, suggesting a B cell-mediated mechanism of disease [6]–[9].