E16.5 hearts in which Hand1 has been ablated and Hand2 function has been reduced by half within the SHF (Hand1fx/fx;Hand2fx/+;Mef2c-AHF-Cre(+)) display a low penetrance of VSDs (B, arrowhead; summarized in Table 3). Coronal sections have been stained with Sirius Red and Fast Green. Scale bars = 250 μm. Ao, aorta; LA, left atrium; RA, right atrium; LV, left ventricle; RV, right ventricle.