TCF4 and bipolar disorder: An expansion of a CTG TNR, located in an alternative promoter region between TCF4 exons 3 and 4 (known as CTG 18.1), upstream of TCF4 5′ exons 4a, 4b and 4c causes Fuchs' endothelial corneal dystrophy (FECD) and has been linked to Bipolar disorder (BD)14,15.