The Antennapedia complex (ANT-C) of Drosophila is a cluster of genes involved in the regulation of morphogenesis, including at least three homoeotic loci (Antp, Scr, and pb), in which mutations cause switches of cell fates from one developmental pathway to another. We have isolated DNA clones containing most of the ANT-C, a region of about 300 kb. Antp mutations are distributed across more than 100 kb of the ANT-C. Dominant Antp mutations are associated with certain chromosome rearrangements and insertions that interrupt the 100 kb region; other chromosome breaks within the region cause recessive lethality and have no dominant effects. Two prominent transcripts (3.5 and 5.0 kb) are derived from exons within and at the two ends of the 100 kb region.