Hox gene
The ancestors of vertebrates had a single Hox gene cluster, which was duplicated (twice) early in vertebrate evolution by whole genome duplications to give four Hox gene clusters: Hoxa, Hoxb, Hoxc and Hoxd. It is currently unclear whether these duplications occurred before or after divergence of lampreys and hagfish from the rest of vertebrates. Most mammals, amphibians, reptiles and birds have four HOX clusters, while most teleost fish, including zebrafish and medaka, have seven or eight Hox gene clusters because of an additional genome duplication which occurred in their evolutionary history. In zebrafish, one of the eight Hox gene clusters (a Hoxd cluster) has lost all protein-coding genes, and just a single microRNA gene marks the location of the original cluster. In some teleost fish, such as salmon, an even more recent genome duplication occurred, doubling the seven or eight Hox gene clusters to give at least 13 clusters