Axial <i>Myod</i> expression is lost in <i>Myf5^−/−:Six1^−/−/Six4^−/−</i> embryos.

<p>A–H, Whole mount <i>in situ</i> hybridization using a <i>Myod</i> probe, I–L, X-Gal staining, and i′–l′, i″–l″ immunohistochemistry on sagittal sections of I–L embryos at the interlimb somites or head level using Desmin antibodies, at E11.5 (A–H) or E12.5 (I–L) with <i>Pax3^+/SpSix1^+/−Six4^+/−</i> (A), <i>Pax3^+/SpSix1^−/−Six4^−/−</i> (B) <i>Pax3^Sp/SpSix1^+/−Six4^+/−</i> (C), <i>Pax3^Sp/SpSix1^−/−Six4^−/−</i> (D), <i>Myf5^+/−Six1^+/−Six4^+/−</i> (E, I), <i>Myf5^−/−Six1^+/−Six4^+/−</i> (F, J) <i>Myf5^+/−Six1^−/−Six4^−/−</i> (G, K), <i>Myf5^−/−Six1^−/−Six4^−/−</i> (H, L) embryos, showing the role of Pax3/Six proteins and Myf5 acting upstream of <i>Myod</i> during trunk myogenesis. Desmin expression in E12.5 compound embryos at the axial level (i′–l′) and at the head level (i″–l″) is not detected in <i>Myf5^−/−Six1^−/−Six4^−/−</i> embryos at the axial level (l′) but at the head level (l″), showing that craniofacial myogenesis can take place in this compound mutant. e: eye. White arrow in L shows the presence of craniofacial muscles.