The Hox gene clusters and their effects on development. (a) Each Hox gene in the fruit fly directs the development of structures in a defined part of the body and is expressed in defined regions of the embryo (coded by color). (b) Drosophila has one Hox gene cluster (HOM-C). The genes are color-coded to match the fly segments in (a). The human genome has four Hox gene clusters (Hox-A through Hox-D). Many Hox genes are highly conserved in animals. Evolutionary relationships between genes in the Drosophila Hox gene cluster and those in the human (mammalian) Hox gene clusters are indicated by dashed lines. Similar relationships between the four sets of human Hox genes are indicated by vertical alignment.