PTPN22 and rheumatoid arthritis: Moreover, RA-associated allelic variants of several genes like protein tyrosine phosphatase non-receptor type 22 (PTPN22), cytotoxic T-lymphocyte-associated protein 4 (CTLA4), the zeta chain of T cell receptor-associated protein kinase 70 (ZAP70), and peptidyl arginine deiminase 4 (PADI4) encode molecules involved in the T cell activation pathways.