Fig. 7. b-Catenin alleles and Mef2c^cre lineage tracing. (A) Schematic structure of Mef2c AHF^cre transgene (abbreviated as Mef2c^cre elsewhere). Cre recombinase was driven by the anterior heart field-specific regulatory promoter and enhancer of mouse Mef2c gene and terminated by SV40 polyadenylation sequences. (B-I ) Mef2^cre fate mapping b-gal staining. Mef2c^cre mice were crossed with Rosa26Reporter mice, and embryos were harvested for whole-mount b-gal staining. Shown are ventral (B, C, D, and F) and right side (H) views of different stage mouse embryos to show cre activity in SHF and derivatives. Transverse sections of 8.0-9.5 dpc embryos (E, G, and I) showing the Cre activity in pharyngeal arch mesoderm, outflow tract myocardium, endocardium (red arrowhead), right ventricle, and rightmost region of left ventricle. (J) Schematic structure of b-catenin^flox allele (b-catenin^flox). Exon 2-6 were floxed by LoxP sites, and exon 2 contains transcriptional start codon ATG, so cre-induced recombination results in a b-catenin null allele. (K) Schematic structure of b-catenin^E3flox allele (b-cat^E3flox). Floxed exon 3 contains four phosphorylation sites that target b-catenin to the degradation pathway. After cre-induced recombination, b-catenin will be stabilized, allowing for cytoplasmic accumulation and nuclear translocation to initiate target gene transcription. SHF is denoted by yellow arrowheads, OFT is denoted by black arrowheads, and endocardium is denoted by red arrowheads. SHF, second heart field; HF, head fold; HT, heart tube; OFT, outflow tract; PM, pharyngeal mesoderm; P, pharynx; RV, right ventricle; LV, left ventricle.