Approximately 40% of genetic susceptibility to RA is accounted for by the HLA-DRB1 alleles encoding the shared epitope (SE), the major RA susceptibility locus [3], together with the protein tyrosine phosphatase nonreceptor 22 gene (PTPN22), a second susceptibility gene confirmed in populations of northern European ancestry [4]. The gene discussed is PTPN22; the disease is rheumatoid arthritis.