PTPN22 and rheumatoid arthritis: The protein tyrosine phosphatase non-receptor type 22 (PTPN22) gene encodes the lymphoid-specific tyrosine phosphatase (Lyp) and a genetic single-nucleotide polymorphism of PTPN22 rs2476601 (R620W) has been associated with several human autoimmune diseases, including rheumatoid arthritis (RA).