H2BC21 and rheumatoid arthritis: Notably, a number of citrullinated residues that we detected in periodontal samples have previously been identified in synovial fluid from RA patients (R35 in fibrinogen α [32], R715 in α2‐macroglobulin, R450 and R304 in vimentin, and R93 in histone H2B type 2‐E [33]).