AQP4 and rheumatoid arthritis: Anti-AQP4 antibody is closely associated with occurrence of NMOSD,11 the proportion of positive anti-AQP4 antibody and the titer of anti-AQP4 antibody were all increased in NMOSD, but this antibody was not detected in SLE and RA patients in a previous study.12 Regarding the genetic background, the HLA-DRB1 *04 allele increases susceptibility to seropositive RA and NMOSD,13,14 and we speculate that a common genetic locus leads to the simultaneous occurrence of both diseases.