Around E11.5, the aortic sac normally forms a distinctive T-shaped structure, as seen in frontal sections of the control sample in Fig. 3(A,C). Subsequently, the right horn of this structure transforms into the prospective brachiocephalic artery, while the left horn together with the left 4th PAA gives rise to the definitive aortic arch [23]. In Alk5/Wnt1-Cre mutants, the T-shaped aortic sac failed to form (Fig. 3B,D). Instead, the truncus bifurcated to a left and right arm, which further branched to the PAAs, particularly to the predominant pair of 6th PAAs (Fig. 3B,D). The observed phenotype is consistent with the absence or severe hypoplasia of structures derived from the aortic sac in late stage embryos (E17), e.g., the missing brachiocephalic artery and severe shortening of the ascending truncus as shown in the Figure 1.