Fig. 9. Model for the role of the Raf/MEK/ERK signaling pathway in the gene network regulating mesendoderm formation. At the blastula stage, activation of the Raf/MEK/ERK signaling pathway by an unknown factor Y, allows phosphorylation of Ets1, which is crucially required for the maintenance of alx1 and Delta expression in the micromeres, and for the initiation of skeT/Tbr and other downstream genes in this lineage.