6q23 is one of the most important loci for RA susceptibility, being the third most strongly associated region after HLA-DRB1 and PTPN22. Although T-cells are thought to be the most important cell type in RA pathogenesis, synovial fibroblasts have also been shown to play a crucial role in the perpetuation of disease [35]. Here, PTPN22 is linked to rheumatoid arthritis.