Nkx2.5 null mice died from cardiac malformations [1], [2], while mice with Nkx2.5 haploinsufficiency caused atrial septal defects (ASDs), one of the most common forms of congenital heart diseases (CHDs) in humans, although the penetrance was low and was influenced by the mouse strain [3], [4]. The gene discussed is NKX2-5; the disease is atrial septal defect.