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.