Hox gene
When gene duplication occurs to produce a gene cluster, one or multiple genes may be duplicated at once. In the case of the Hox gene, a shared ancestral ProtoHox cluster was duplicated, resulting in genetic clusters in the Hox gene as well as the ParaHox gene, an evolutionary sister complex of the Hox gene. It is unknown the exact number of genes contained in the duplicated Protohox cluster; however, models exist suggesting that the duplicated Protohox cluster originally contained four, three, or two genes.