We tested 112 sera, which included 40 NMO sera, on cells transfected with: 1) AQP4-M23 expressing vector producing only the OAP forming AQP4-M23 isoform, 2) AQP4-M1 expressing vector, having a WT TIS (G−3) and therefore producing AQP4-M1>AQP4-M23 and 3) a AQP4-M1 mutant (M1M23I) exclusively producing AQP4-M1 since AQP4-M23 synthesis via LS is prevented (Fig. 1A) [23]. Here, AQP4 is linked to neuromyelitis optica.