Fig. 1. An interaction between Gbx2 and Otx2 defines the limits of their respective expression domains at the start of somitogenesis. (A) Expression of Gbx2 and Hesx1 in wild-type and Otx2^hOtx1/hOtx1 embryos at the EHF stage. The anterior limit of Gbx2 expression in the midline (arrow), relative to the position of the node (arrowhead) in Otx2^hOtx1/hOtx1 and wild-type embryos is comparable, although the lateral expression of Gbx2 (asterisk) appears slightly expanded anteriorly in the mutant. (B) Gbx2 expression is significantly expanded anteriorly in Otx2^hOtx1/hOtx1 embryos at the LHF stage, compared with wild-type controls. (C,D) Expression of Otx2 in wild-type and Gbx2^–/– embryos at the EHF (C) and LHF (D) stage. Relative to the position of the node (arrowhead), the posterior limit of Otx2 (arrow) is not altered in Gbx2^–/– embryos at the EHF stage, but shifted caudally by the LFH stage, particularly in the midline. (E) Expression of Gbx2 is normal in Gbx2^–/– embryos at the EHF stage. (F) Hesx1 expression (brackets) is not changed in Gbx2^–/– embryos, whereas Gbx2 expression is reduced and its anterior limit (arrow) is shifted caudally compared with wild-type controls at the LHF stage. The apparently stronger staining of Hesx1 in the Gbx2^–/– embryo is due to a prolonged color reaction in the mutant compared with the wild type in order to visualize Gbx2 staining. (B,F) Dorsal views of flat-mount embryos.