AQP4 and synovial sarcoma: Some researchers considered that the coassociated mechanisms of SS and NMOSD may be related to the AQP epitope diffusion hypothesis.[16] AQP4 and AQP5 are structurally homologous, 50% of the gene sequence of AQP4 and AQP5 is common.[17] There is likely to be a common target, resulting in anti-AQP4 antibody binding to both AQP4 and AQP5 in multiple sites of exocrine glands, brain, and other sites, and the subsequent reactions may lead to the coexistence of SS and NMOSD, although the internal mechanism between SS and NMOSD remains to be elucidated.