Like Xenopus oocytes, Xenopus cleavage blastomeres are ideal synthetic factories for the expression of heterologous gene products. For 6 h after fertilization, the embryos are transcriptionally quiescent (1 ), but mRNA is recruited for translation at a rate greater than in the oocyte, and the protein products of exogenous mRNA and DNA, introduced by microinjection, are synthesized as efficiently as those of endogenous RNA (2 ). This is an excellent system in which to examine the developmental function of heterologous genes, because one can express genes during developmentally inappropriate stages and in developmentally inappropriate regions. For example, to test whether a gene is involved in dorsal axis formation, researchers express the gene in a blastomere that produces ventral tissues and assay whether the blastomere’s fate changes to a dorsal one (3 –(6 ). Spatial misexpression is possible in Xenopus because.