H2BC21 and rheumatoid arthritis: A number of the citrullinated sites that we identified in this study have previously been identified in RA synovial tissue or fluid, and demonstrated to act as autoantigen epitopes, including R35 in fibrinogen α 32, R715 in α2‐macroglobulin, R450 and R304 in vimentin, R93 in histone H2B type 2‐E 33, R4 in histone 2A 34, as well as R69 and R71 in vimentin 29.