Fig. 4. Alteration of Phox2a and Phox2b expression in Eya1^–/– and Six1^–/– epibranchial neuronal precursors. Whole-mount hybridization showing Phox2a (A) and Phox2b (B) expression in the epibranchial neurons in wild-type embryos. (C,D) In Eya1^–/– embryos, weak Phox2a and Phox2b expression was only observed in the Xth neurons. (E,F) In Six1^–/– embryos, their expression was absent (E) or markedly reduced in the VIIth (arrowhead in F), markedly reduced in the IXth, and relatively normal in the Xth neurons.