A mutation in a known monogenic autoimmunity gene was identified in 47% (37/79) of the individuals with diabetes and ≥1 autoimmune disorder diagnosed before the age of 5 years; 25 male participants had a hemizygous mutation in FOXP3, eight individuals had recessively inherited mutations in LRBA, two had recessively inherited IL2RA mutations and two had heterozygous gain-of-function STAT3 mutations. This evidence concerns the gene STAT3 and diabetes mellitus.