AQP4 and neuromyelitis optica: This is clinically important given that (a) the M1-DNA-based (‘classic’) assay evaluated here has been used by many laboratories over the past couple of years and employed in many scientific studies on NMO and its spectrum disorders, and (b) some recent studies have suggested that transfection with the shorter, so-called M23 isoform of AQP4 might improve assay sensitivity.