PTPN22 and rheumatoid arthritis: Genome-wide association (GWA) scans have emphasised the importance of the PTPN22 gene, encoding the phosphatase LYP, in susceptibility to rheumatoid arthritis (RA) in European and European-ancestry populations, with HLA and PTPN22 locus SNPs dominating association at the genome-wide level.[1]–[3] The Arg620Trp variant, encoded by SNP rs2476601 (C>T), of the protein tyrosine phosphatase-22 (PTPN22) gene, is a prominent determinant of some autoimmune phenotypes, including RA.