Homeotic genes occupy a central position in the hierarchy of genes controlling the early steps of embryonic development in Drosophila. They are involved in the specification of the individual identity of each segment of the insect's body. Most homeotic genes of Drosophila are located in two large gene clusters, the Antennapedia complex (ANT-C) and the Bithorax complex (BX-C). These genes are expressed in two partially overlapping domains whose positions along the anterior-posterior axis of the organism are colinear with their position within each complex. Five genes in the ANT-C are involved in the specification of the identity of some segments of the head and of that of the first and second thoracic segments. The three genes of the BX-C determine the identities of the posterior compartment of the second and third thoracic segments and of the eight abdominal segments. Molecular studies have revealed that the proteins coded by the homeotic genes share a 60 amino acid motif, the homeo box, whose helix-turn-helix structure enables them to bind as transcription factors to specific DNA sequences in the cis-acting regulatory regions of their target genes. The high degree of phylogenetic conservation of the homeobox has made possible the identification and the isolation of more than fifty homeobox genes (Hox genes) in many eukaryotic organisms including nematode, Xenopus, mouse and man. In the mouse and human genomes Hox genes are clustered in tandem in four large gene complexes located on separate chromosomes. The relative positions of the genes within the clusters and the positions of the anterior boundaries of their expression domains along the anterior-posterior axis of the central nervous system and prevertebral column of the mouse embryo were found to be colinear. The similarities between the structure, organization and pattern of expression of the homeotic genes of Drosophila and of the homeobox genes of vertebrates suggest that some basic principles of embryonic organization and development have been preserved during the long period of evolution that has elapsed since the divergence of the phylogenetic lineages leading to vertebrates and arthropods.