GCG and type 2 diabetes mellitus: Variants in TCF7L2—among the strongest genetic determinants of T2DM—impair β-cell insulin-secretory capacity and thereby influence incretin efficacy; functional studies show that carriers of the rs7903146 risk allele have blunted GLP-1-stimulated insulin release [71], supporting the GWAS findings by Lyssenko et al. [72].