TCF7L2 and type 2 diabetes mellitus: The rs7903146 T allele in TCF7L2 is the strongest GWAS signal for T2DM risk in different populations across the world and it is associated with insulin synthesis, processing, secretion and action mechanisms (Grant et al., 2006; Cauchi et al., 2008b; Bouhaha et al., 2010; Zhou et al., 2014; Corella et al., 2016; Cropano et al., 2017).