A. Diagram showing the chromosomal organization of Hox genes in human. Each horizontal thick line represents a gene cluster, with cluster name shown at the left side. Clusters are shown from 3' end to 5' end. The 13 cognate gene groups are defined vertically across the clusters and consisting of up to four genes, each on a different chromosome. Some genes in each cognate group have been lost during evolution. B. A phylogenetic tree illustrating the evolutionary relationships and approximate divergence times of representative mammalian species. The branch lengths represent the estimated divergence times. The divergence times were obtained from the TimeTree database. MYA: million years ago.