First, it has been suggested that case ascertainment bias [17] may drive the association of TCF7L2 T2D risk alleles and lower BMI, as previous GWAS have shown attenuated effects of TCF7L2 on BMI among population-based samples as compared to the effect sizes in samples of T2D cases [15]. This evidence concerns the gene TCF7L2 and type 2 diabetes mellitus.