First, NMO patients may simply harbor not only M23-specific AQP4-IgG in their serum but generally also an amount of AQP4-IgG binding to M1-AQP4, or both M1-AQP4 and M23-AQP4, sufficient to yield positive test results also in M1-based assays. Here, AQP4 is linked to neuromyelitis optica.