PTPN22 and rheumatoid arthritis: The heritability of RA has been reported as 50–60% (12), and human leukocyte antigen-DR (HLA-DR), protein tyrosine phosphatase non-receptor 22 (PTPN22), tumor necrosis factor receptor-associated factor 1/complement component 5 (TRAF1/C5) and peptidyl arginine deiminase 4 (PADI4) alleles have been implicated in the genetic predisposition.