If the gradual increase in Nkx2.5 expression in the DMP is the driving force behind the muscular differentiation of the DMP mesenchyme after the formation of the AV mesenchymal complex, and if Nkx2.5 expression is indeed elevated in the pSHF of the Pdgfrα knockout mouse, then premature and ectopic myocardial differentiation of the pSHF cell population should be considered as a possible mechanism involved in inhibiting normal DMP formation and causing AVSD. This evidence concerns the gene NKX2-5 and familial atrioventricular septal defect.