Using single‐cell sequencing and monoclonal antibody expression of B‐cells from the joints of ACPA+ RA patients with active disease, 25% of synovial IgG‐expressing B‐cells were specific for citrullinated autoantigens; however, these were not found in ACPA‐negative RA patients.109 While some of the ACPAs bound more than one citrullinated antigen, none was reactive to non‐citrullinated antigen. This evidence concerns the gene PRTN3 and rheumatoid arthritis.