AQP4 and neuromyelitis optica: Thirdly, in previous studies, there was a large amount of heterogeneity among the NMO or NMOSD patients and various types of patients were included, such as AQP4-Ab seropositive NMOSD or NMO, AQP4-Ab seronegative NMOSD or NMO, and longitudinal extensive transverse myelitis with AQP4-Ab seropositivity[16–18, 20].