TCF7L2 and type 2 diabetes mellitus: Representative observations include (i) a strong association of T allele of the transcription factor 7-like 2 (TCF7L2) Rs7903146 polymorphism with β-cell dysfunction in T2D (Lyssenko et al., 2007), (ii) many observations on the decline in functional β-cell mass as a hallmark trait of T2D, and (iii) significantly lower β-cell mass, smaller islets, and fewer islet equivalents in the pancreas from T2D postmortem studies.