Homeotic genes switch the developmental fates of cells between alternative pathways of development. The title was once reserved for a few loci in the fruit fly Drosophila melanogaster, which, when mutated, produced bizarre transformations of appendages. But as the phenotypic effects of more mutations in various organisms are described at the cellular level, many other genes are being recognized as homeotic switches. The isolation of the DNA for these loci is now uncovering the molecular mechanics of developmental decisions. The DNA sequences for two such homeotic loci -the Notch locus in Drosophila and the
lin-12 locus in the soil nematode, Caenorhabditis elegans-are presented in this issue of Cell. There was no reason, until now, to compare these apparently unrelated genes in manifestly unrelated organisms. The surprise is that both loci encode proteins with homologies to epidermal growth factor (EGF).