THE Brachyury (T) gene is required cell-autonomously for mesoderm formation in the posterior of the mouse embryo^1,2, and both its complementary DNA sequence and expression pattern³closely resemble those of a Xenopus homologue (Xbra)⁴, suggesting that these genes have an evolutionarily conserved function in vertebrate development. Strong expression of Xbra messenger RNA is found in the ring of involuting mesoderm during Xenopus gastrulation⁴, and the expression of Xbra is an immediate–early response of animal pole blastomeres to mesoderm-inducing factors⁴. To assess the role of Xbra in mesoderm formation, we increased its domain of expression in the embryo by microinjection of Xbra transcripts into the animal pole of Xenopus embryos at the one-cell stage. We show that expression of Xbra by cells of the early embryo is sufficient to direct their development into differentiated mesodermal tissues. At the molecular level this response shows a sharp threshold of sensitivity to the dose of Xbra RNA delivered, and we suggest that Xbra may act as a genetic switch initiating posterior mesodermal specification during embryogenesis.