HLA-DRB1 and rheumatoid arthritis: Candidate gene approaches, genome-wide association studies (GWAS), and trans-ethnic GWAS meta-analyses have identified a number of RA risk genes, such as HLA-DRB1, PTPN22, STAT4, CCR6, TNFAIP3, PADI4, CD40, and FCRL3, many of which are involved in the functions of immune cells, including T cells, B cells, and macrophages [38, 39].