The exact cause of RA is still unknown, but genetic factors, such as human leukocyte antigen-DR4 (HLA-DR4) and other non-HLA genes including protein tyrosine phosphatase, non-receptor type 22 (PTPN22) and peptidyl arginine deiminase, type IV (PADI4), are suspected as major contributing factors [7, 8]. Here, PTPN22 is linked to rheumatoid arthritis.