The body plan of Drosophila is determined to a large extent by homeotic genes, which specify the identity and spatial arrangement of the body segments. Homeotic genes share a characteristic DNA segment, the homeo box, which encodes a defined domain of the homeotic proteins. The homeo domain seems to mediate the binding to specific DNA sequences, whereby the homeotic proteins exert a gene regulatory function. By isolating the normal Antennapedia gene, fusing its protein-coding sequences to an inducible promoter, and reintroducing this fusion gene into the germline of flies, it has been possible to transform head structures into thoracic structures and to alter the body plan in a predicted way. Sequence homologies suggest that similar genetic mechanisms may control development in higher organisms.