Embryo development requires a complex order of events that must occur at the correct time and in the correct space. A series of decisions takes place as cells divide and become committed to increasingly specialized and limited domains of the embryo. Many genes that are important in orchestrating this process were originally identified in the fruitfly Drosophila melanogaster. Conserved sequences from the fly genes have been used to clone homologous genes in vertebrates, including mice. Studies of the pattern of expression of these genes during murine development in conjunction with the use of new functional assays suggest that not only DNA sequences, but also functional roles during embryogenesis, have been conserved. Thus, we may have the tools in hand to begin to understand how vertebrate development and cell differentiation take place.