Measurement of demethylation at the T-reg specific demethylated region (TSDR), a specific marker of thymic derived regulatory T cells [17], represented as a proportion of all T helper cells (as measured by demethylation at a CD4 T helper cell specific demethylated region) could be useful for interpreting novel variants of uncertain significance in FOXP3, facilitating early diagnosis where clinical features are not sufficiently informative (i.e., patients have isolated diabetes). This evidence concerns the gene FOXP3 and diabetes mellitus.