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Fig. 2. Tcf1 and Tcf3 are non-redundantly required for mesoderm induction. Vegetal view of brachyury (Xbra) expression in stage 10.5 embryos, dorsal towards the top, injections into the right side. Xbra expression is not affected by injection of 60 ng control MO (A), Lef1 MO (B) or Tcf4 MO (C), but is completely blocked by injection of 20 ng Tcf1 MO (D) and significantly downregulated by injection of 60 ng Tcf3 MO (G) in the injected tissue. Blocking of Xbra expression by XlTcf1 MO is rescued by co-injection of 0.3 ng XtlTcf1 mRNA (E), but is not rescued by injection of 0.5 ng HA-Tcf3 mRNA (F). Downregulation of Xbra expression by XlTcf3 MO is rescued by co-injection of 0.5 ng HA-XlTcf3 mRNA (I), but is not rescued by co-injection of 0.3 ng XtlTcf1 mRNA (H). (J-N) Numerical summary illustrating penetrance of effects of Tcf/Lef MOs and Tcf/Lef mRNA on Xbra expression in stage 10.5 embryos, indicating absent, reduced and normal or almost normal Xbra expression detected at the site of injection. Control MO, Lef1 MO and Tcf4 MO do not significantly affect Xbra expression (J). mRNA injection-mediated overexpression of relatively low amounts of Tcf1 or Tcf3 hardly affects Xbra expression, while higher amounts of either Tcf1 or Tcf3 inhibit Xbra expression more dramatically and in a dose-dependent way (K). Xbra expression is strongly inhibited by Tcf1 MO in the vast majority of injected embryos, but is rescued by co-injection of relatively low amounts of Tcf1 mRNA; however, increasing amounts of Tcf1 mRNA result in an apparently less successful rescue, and Tcf3 mRNA fails to rescue Xbra expression altogether (L). Xbra expression is reduced by Tcf3 MO in the vast majority of injected embryos, but is dramatically rescued by relatively low amounts of Tcf3 mRNA and hardly rescued by relatively low amounts of Tcf1 mRNA; however, increasing amounts of Tcf3 or Tcf1 mRNA result in an apparently less successful rescue (M). At relatively low doses, Tcf1 MO or Tcf3 MO also inhibit Xbra expression to a lesser degree; however, Xbra expression is not further inhibited by co-injection of these two MOs, but is rescued to some extent (N).