Hox genes are a subset of homeobox genes that specify regions of the body plan of an embryo along the head-tail axis of animals. They are a group of related genes that encode and specify the characteristics of position, ensuring that the body parts develop in the correct location. The protein product of each Hox gene is a transcription factor, and each Hox gene contains a well-conserved DNA sequence known as the homeobox. Hox genes play critical roles in the development of structures such as limbs, lungs, the nervous system, and eyes.

Hox genes are not the only genes to possess a homeobox sequence, but they are a subset of the homeobox transcription factor genes. Humans have over 200 homeobox genes, of which 39 are Hox genes. Hox gene clusters are thought to have developed by a process of duplication and divergence from a primordial homeobox gene estimated to have arisen about 1,000 million years ago.

Hox genes are found in almost all animals, generally organized in large clusters of up to 15 genes. Their amino acid sequence has been used to classify Hox proteins in distinct homology groups and to infer how the cluster evolved. Hox proteins can bind DNA as monomers or in complex with cofactor proteins. Due to the convergent recruitment of Hox proteins into cephalization after the split of both lineages, it is unlikely that they regulate similar downstream targets or gene networks in vertebrates and invertebrates.

In summary, Hox genes are a subset of homeobox genes that play a critical role in the development of animal body plans along the head-tail axis. They encode transcription factors that ensure that body parts develop in the correct location. Hox genes are found in almost all animals and are generally organized in large clusters of up to 15 genes.