PTPN22 and type 1 diabetes mellitus: PTPN22 R620W, hereafter referred to as the PTPN22 risk variant, is one of the strongest, non-HLA, genetic risk factors for autoimmune diseases, nearly doubling a carrier’s risk of type 1 diabetes, systemic lupus erythematosus, or rheumatoid arthritis (Bottini et al., 2004; Begovich et al., 2004; Kyogoku et al., 2004).