H2BC21 and rheumatoid arthritis: First, we screened the RA-rmAbs in a synovial autoantigen microarray platform27 and in a multiplex RA-associated citrullinated antigen assay.5 Strikingly, RA-rmAbs (∼40%) showed strong immunoreactivity towards citrullinated histones H2A (citH2A) and citH2B by multiplex assay (figure 3A) with reactivity to histones H2A and H2B also frequently observed in the protein array heatmap (see online supplementary figure S3).