The misexpression and overexpression of proteins by injection of synthetic mRNA into Xenopus oocytes and, more particularly, embryos has provided the experimental basis for a considerable portion of our current knowledge of early vertebrate development (1 , and references therein). The original experiments performed were designed to test the function of proteins in early development by expressing large quantities of the protein itself both in the cells and at the times when it would normally be expressed (overexpression) and in cells or at times when it would not (ectopic expression). These experiments was used to test the roles of both transcription factors and signaling molecules in Xenopus embryos.