TCF7L2 and diabetes mellitus: Given the existing controversy in the literature over the relative importance of Tcf7l2 for proper development of β cells, liver and/or other tissues, as well as the contributions of extra-pancreatic tissues to risk variants activity on diabetes susceptibility17, the present study has been designed to investigate the general effects of Tcf7l2 in a simple model: the zebrafish (Danio rerio).