AQP4 and neuromyelitis optica: In most cases, NMO-IgG preferentially binds to cells expressing M23 rather than to those expressing M1 alone, implying that OAP-formation of AQP4 contributes to recognition by NMO-IgG since the primary sequences of ECDs of M1 and M23 are identical [13], [14], [15], [16], [17], [18].