Homeotic genes of the Antennapedia and bithorax complexes control Drosophila development by encoding DNA-binding proteins that regulate the transcription of target genes. Because either the presence or absence of these DNA-binding proteins alters development, regulation of the spatial patterns of expression is crucial to normal development. Numerous gene products are required for properly regulated expression of Antennapedia and bithorax complex genes, but few (if any) are dedicated solely to the regulation of these genes. One of the pleiotropic activators of homeotic genes in Drosophila, the brahma gene, encodes a protein similar to a yeast protein that is required for transcriptional activation of multiple tightly regulated genes. Other components of this system may be conserved as well, suggesting that the biochemical basis for induced gene expression in single-celled organisms may have more in common with programmed developmental pathways in multicellular organisms than previously thought.