In RA, several SNPs have been reported in HLA class 2 histocompatibility antigen, DRB1 beta chain (HLA-DRB1), protein tyrosine phosphatase non-receptor type 22 (PTPN22), cytotoxic T-lymphocyte-associated protein 4 (CTLA4), and cluster of differentiation 40 (CD40) (McInnes and Schett, 2011; Yarwood et al., 2014; Smolen et al., 2016). Here, PTPN22 is linked to rheumatoid arthritis.