Model for Tbx1 and <i>β</i>-catenin function in the SHF.

<p>In the model, the triangle represents the <i>Mef2c-AHF-Cre</i> lineage and the <i>Tbx1</i> expression pattern, before migrating into the heart tube at E9.5. Left panel: <i>Tbx1</i> expression is strongest in the AHF and weakest in the posterior SHF (pSHF) while <i>Wnt/β-catenin</i> expression is opposite. Left panel depicts a possible double negative feedback loop between the two genes in the SHF, required for normal cardiac outflow tract (OFT) formation. Middle panel shows increased differentiation in the AHF when <i>Tbx1</i> is inactivated or <i>β-catenin</i> is constitutively active in the AHF. This results in premature differentiation within this tissue. Right panel depicts the rescue genotype in which both alleles of <i>Tbx1</i> and one allele of <i>β-catenin</i> was inactivated in the AHF. Significant rescue of heart defects was obtained. Abbreviations: A = anterior, P = posterior, OFT = outflow tract.