AQP4 and neuromyelitis optica: In fact, recent affinity studies using AQP4-transfected human astrocyte-derived U87MG cells found binding to both isoforms, though consistently stronger binding to M23 with wide variations in NMO-IgG/AQP4-Ab binding intensity to M1- versus M23-AQP4 among patients and even among recombinant monoclonal AQP4-Abs generated from different plasma cell clones of a single patient [35].