As described above, Alk5/Wnt1-Cre mutants displayed an inadequate amount of AP-septal tissue in the base of the aortic sac between the origins of 4th and 6th PAAs. To analyze whether this phenotype resulted either from defective CNCC proliferation or inappropriate apoptosis, we used BrdU and TUNEL staining, respectively. While CNCC proliferation was not affected in Alk5 mutants (data not shown), we could detect a dramatic increase in the number of TUNEL positive cells in tissues surrounding the aortic sac including the site where the AP-septum forms (Fig. 7A–C). Dual staining for lacZ and TUNEL positive cells demonstrated that these cells were postmigratory CNCCs; this phenotype was already clearly detectable at E10.5. These results were confirmed by using immunostaining for cleaved caspase-3, another marker for apoptosis (Fig. 7I,J). In the chick, apoptotic neural crest-derived cells have also been found at the sites, where the prongs of the AP septum penetrate into the OFT cushion mesenchyme [25,26]. Thus, we compared apoptosis patterns also on the more proximal level, but found no detectable differences at E11.0 between Alk5/Wnt1-Cre mutants and controls (Fig. 7D,E). Unlike in Alk5/Wnt1-Cre mutant embryos, increased apoptosis of NC-derived cells is not responsible for the observed defects in the OFT septation in corresponding Alk2 mutants (Fig. 7C,E).