The production of transgenic laboratory animals has rapidly gained importance as an experimental tool to study the factors that determine the tissue specificity of gene expression, to generate models for human diseases (1,2), as well as to look at the consequences of oncogene expression (3,4). Through this technique, recombinant DNA molecules of any type can be introduced into zygotes for use in producing transgenic domestic animals. Introduction of foreign genes into early embryos, particularly into bovine embryos, may be a powerful tool to obtain more valuable animals. Although pronuclear microinjection is the only proven route for gene transfer in livestock, the production of transgenic animals is still problematic and inefficient (5), and importantly, microinjection may impair the development of the embryos (6).