Hox gene
Homeobox genes, and thus the homeodomain protein motif, are found in most eukaryotes. The Hox genes, being a subset of homeobox genes, arose more recently in evolution within the animal kingdom or Metazoa. Within the animal kingdom, Hox genes are present across the bilateria (animals with a clear head-to-tail axis), and have also been found in Cnidaria such as sea anemones. This implies that Hox genes arose over 550 million years ago. In bilateria, Hox genes are often arranged in gene clusters, although there are many exceptions where the genes have been separated by chromosomal rearrangements. Comparing homeodomain sequences between Hox proteins often reveals greater similarity between species than within a species; this observation led to the conclusion that Hox gene clusters evolved early in animal evolution from a single Hox gene via tandem duplication and subsequent divergence, and that a prototypic Hox gene cluster containing at least seven different Hox genes was present in the common ancestor of all bilaterian animals.