Fig. 4. Xiro1 participates in the positioning of the isthmus organizer. Embryos were injected in one blastomere at the two-cell stage with 2 ng of Xiro1 mRNA (A,D,G,J), 0.5 ng of HD-GR-EnR (B,E,H,K) or HD-GR-E1A (C,F,I,L); the inducible constructs were induced around stage 12.5. The injected side is marked by X-Gal stain in the Xiro1-injected embryos and by Myc staining in the case of the inducible constructs. (A) Xiro1 overexpression promotes an expansion and caudal shift of Gbx2. (B) HD-GR-EnR mRNA injection causes expansion and anterior shift of Gbx2 expression. (C) Gbx2 is repressed in embryos injected with HD-GR-E1A mRNA. (D) In embryos injected with Xiro1 mRNA Otx2 midbrain expression domain is expanded caudally. (E) However, injection of HD-GR-EnR mRNA caused an anterior shift of the Otx2 expression domain. (F) A caudal expansion of Otx2 when HD-GR-E1A mRNA is overexpressed. (G) Fgf8 expression is displaced posteriorly in embryos injected with Xiro1 mRNA. (H) Overexpression of HD-GR-EnR promotes an expansion and anterior shift of the isthmus domain of Fgf8. (I) This domain is repressed in HD-GR-E1A-injected embryos. (J) In embryos injected with Xiro1 mRNA, Pax2 is expanded. (K) HD-GR-EnR mRNA injection causes an anterior shift of Pax2 expression. (L) Pax2 is repressed and shifted caudally in embryos injected with HD-GR-E1A mRNA. Broken lines show the described effects. Arrowheads indicate the injected sides. Each experiment was performed at least twice with a minimum of 45 embryos. The percentage of effect for each experiment was 
70%.
