Hox gene

Hox genes are a special family of homeotic genes that act like mileposts along the body's main axis, telling cells how far they are from head to tail and therefore what to build there — neck here, ribcage there, tail at the end. They are the address system that gives an animal its front-to-back order.

Remarkably, Hox genes are arranged in clusters on the chromosome in the same order as the body regions they control, a property called colinearity: genes at one end of the cluster pattern the head, and genes further along pattern progressively more posterior regions. They are also expressed in roughly that order in time and space, so the chromosomal layout mirrors the anatomy.

The same Hox clusters, sharing the homeobox, are found across animals from insects to humans, which is among the strongest evidence that diverse body plans were built by tinkering with a deeply conserved ancient toolkit. Changes in where and when Hox genes act underlie major differences such as the number of segments bearing legs or wings.