Fig. 6. Model for the roles of XTcf3 in regulating dorsal gene expression in the Xenopus blastula and gastula; XTcf3 acts solely as a repressor of organizer genes. (A) In the wild-type embryo, XTcf3 represses Wnt targets on the ventral side and in animal cap cells. Repression is alleviated dorsally by the action of stabilized ß-catenin. VegT is also required for activation of goosecoid (gsc) and Xnr6. (B) In the absence of XTcf3, siamois (sia) and Xnr3 are activated dorsally, ventrally and in animal cells via unknown activators. Genes such as gsc and Xnr6 still require VegT for their activation.

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