In early GWAS, peptidyl arginine deiminase 4 (PADI4), protein tyrosine phosphatase non-receptor type 22 (PTPN22), and cytotoxic T-lymphocyte-associated protein 4 (CTLA4) genes were identified as initial non-HLA RA risk loci [29,30] (Figure 2), with these findings being subsequently replicated in multiple cohorts in the following years [28]. Here, PTPN22 is linked to rheumatoid arthritis.