PADI4 and rheumatoid arthritis: Well-documented upstream genetic factors implicated in RA pathogenesis include both human histocompatibility antigens (i.e., human leukocyte antigen, HLA, such as HLA-DR4, HLA-DRB1) and non-HLA genes (PTPN22, CTLA-4, TRAF1-C5, STAT4, PADI4) [3].