The association between several genes and metabolic diseases has been identified by the candidate gene approach and genome-wide scans; to date, besides the FTO (Fat mass and obesity associated) gene, the strongest obesity risk loci known so far [1, 3, 7, 8], two commonly studied candidates for obesity and T2D have been the Melanocortin 4 Receptor (MC4R) and Transcription Factor 7-Like 2 (TCF7L2) genes. The gene discussed is TCF7L2; the disease is type 2 diabetes mellitus.