PTPN22 and rheumatoid arthritis: Many genes contribute to increase the susceptibility to rheumatoid arthritis [2], including human leukocyte antigen (HLA) (such as HLA-DRB1) [3] coding genes and non-HLA coding genes (such as Protein tyrosine phosphatase non-receptor type 22 (PTPN22), Peptidylarginine deiminase 4 (PADI4)) [4–6].