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. Here, TCF4 is linked to Behcet disease.