TCF7L2 and type 2 diabetes mellitus: While human genetics and genomics data suggest that increased T2D risk is associated with increased TCF7L2 expression, a notion supported by various genetically engineered mouse models, other studies focused on the beta cell roles of TCF7L2 have pointed in the opposite direction, suggesting that a TCF7L2 loss of function in beta cells may represent the causal link between genetic variation in the TCF7L2 locus and T2D risk.