PTPN22 and rheumatoid arthritis: So far, using genome-wide association studies (GWAS), research studies have revealed that several genetic susceptibility loci play a vital role in RA, such as the human leukocyte antigen D-related B1 gene (HLA-DRB1) and protein tyrosine phosphatase non-receptor type 22 (PTPN22) (Tanaka, 2020).