PTPN22 and type 1 diabetes mellitus: In addition, a number of T1D susceptibility genes identified prior to the WTCCC study have been identified and more thoroughly replicated, including the HLA class II genes INS, CD25, CTLA4, PTPN22, and IFIH1. Given that the SNPs genotyped in the WTCCC study were also genotyped in the HLC, we examined the extent to which the T1D SNPs identified in the WTCCC study were associated with the expression traits corresponding to the genes implicated in the study.