HNF4A and type 2 diabetes mellitus: Beginning with small studies that showed rare variants in MODY genes to have effects on T2D risk in the general population [44]—albeit with penetrances much lower than might have been expected—and continuing with larger studies that provided stronger statistical evidence of association [27, 44, 45••, 46, 47], MODY genes have been consistently shown to harbor not only rare variants that cause early onset Mendelian diseases but also a broader “allelic series” of variants that predispose to the later onset form of T2D.