Considering that the Plexin-A1 level was much higher in RA SFs compared with OA SFs, Semaphorin 5A and Plexin-A1 co-located on the membrane of SFs, we predicted that Plexin-A1 may be a more specific receptor for Semaphorin 5A in RA SFs. This evidence concerns the gene PLXNA1 and rheumatoid arthritis.