PTPN22 and type 1 diabetes mellitus: However, by examining a separate group of age-matched C1858C T1D and T1D patients homozygous for the C1858T PTPN22 variant, a modest trend toward increased transitional and naïve B cells was found in subjects that were homozygous for the C1858T PTPN22 variant, suggesting that the variant contributes to the diabetic phenotype [20].