TCF7L2 and diabetes mellitus: Given the existing controversy in the literature over the relative importance of Tcf7l2 in the β cell versus the liver and other tissues (30,31), and the contributions of extrapancreatic tissues to the action of risk variants on diabetes risk (47), the present study was designed to achieve highly selective deletion of Tcf7l2 in the β cell, from the earliest possible stages in the establishment of a definitive β cell status, i.e. expression of the insulin gene.