Fig. 7. Model of Hh signaling during OFT development. Pharyngeal-derived (PE) SHH performs three main roles. First (1), SHH acts as a direct survival factor to CNCCs. Second (2), in AHF-derived myocardium, SHH acts via Smo to provide a patterning signal that is required for an unknown function in completing septation between E10.5 and E12.5. Third (3), SHH is a direct survival factor for the pharyngeal endoderm, and its loss results in the absence of a secondary signal ("X") necessary for AHF survival and for the lengthening of the OFT. Disruption of any of these processes either alone or in combination can result in a single-OFT phenotype, via different mechanisms. RV, right ventricle.