PTPN22 and rheumatoid arthritis: Another missense risk variant in RA may be the R620W mutation in Protein tyrosine phosphatase, non-receptor type 22 (PTPN22) (derived from 1858C→T-cells) [8], encoding protein tyrosine phosphatase, an enzyme expressed in hematopoietic cells that negatively regulates antigen receptor signaling in B- and T-cells [9].