NKX2-5 and atrial septal defect: The most common phenotype resulting from mutations in either NKX2-5 or MYH6 is secundum ASD.15,29,38 As this patient has the same diagnosis, double heterozygocity could at least partially explain the development of the defect in the proband and the incomplete penetrance of the L122P and A1443D mutations in the parents.