TBX20 and ventricular septal defect 1: In humans, loss-of-function mutations in TBX20 can cause dilated cardiomyopathy, atrial septal defects, or mitral valve disease, while gain-of-function mutations in TBX20 have been reported in patients with Tetralogy of Fallot (i.e., pulmonary outflow tract obstruction, ventricular septal defect, overriding aortic root and right ventricular hypertrophy) [7, 8, 22–24].