Eighteen T2DM gene loci have been isolated in genome wide scans (GWAS), the best replicated of which are melanocortin receptor-4 (MC4R), T-cell factor 7-like 2 (TCF7L2), and peroxisome proliferator-activated receptor gamma (PPARG) (Stumvoll et al., 2005; Ridderstråle and Groop, 2009) genes, but each gene with relatively small effect size (ORs 0.8–1.3). Here, TCF7L2 is linked to type 2 diabetes mellitus.