In a very elegant study, Goddeeris and colleagues showed that conditional deletion of Shh with a Nkx2-5-Cre mouse, directing expression of cre-recombinase to a variety of tissues including the pharyngeal endoderm, resulted in single outflow tract (i.e., persistent truncus arteriosus; PTA), as well as a fully penetrant AVSD phenotype [65], defects that were also reported by Washington Smoak and co-workers to be present in the Shh knockout mouse [66]. The gene discussed is SHH; the disease is familial atrioventricular septal defect.