A class of homeotic genes that control development of structures along the head-to-tail (anteroposterior) axis of a wide range of animals. The Hox genes are organized into clusters on certain chromosomes; jawed vertebrates, for example, have four Hox gene clusters. In mammals these four clusters are designated Hox A, Hox B, Hox C, and Hox D, each on a separate chromosome, with individual genes given numbers, hence, A1, A2, B1, B2, etc. Nematodes, arthropods, and cephalochordates have a single cluster. Hox genes are highly conserved, showing remarkable similarity of DNA sequence and function; each falls into one of several groups of paralogous genes, derived by duplication of ancestral genes. Moreover, in embryos of all animals studied, the Hox genes show colinearity—their sequence of expression in body segments from head to tail reflects their linear arrangement in the homeotic gene clusters.