The homeobox is a conserved sequence motif of about 180 base pairs found in homeobox genes. It encodes the homeodomain, which is a globular DNA-binding domain of about 60 amino acids. Homeodomain proteins are transcription factors that play major roles in many developmental processes of animals and plants by regulating the expression of other genes during development and differentiation, and are responsible for mating-type switching in yeast. Thousands of homeobox genes have been identified in many different species, and they can be grouped into many different classes and families. Often other conserved protein domains are found linked to a homeodomain. In a number of cases, particular classes of homeobox genes lie next to each other on the chromosome and form clusters. The best-known cluster, that is, the HOX cluster, is found in all bilaterian animals. Vertebrates contain four HOX clusters that arose through duplication in early vertebrate evolution; its genes are called Hox genes. Lower chordates, insects, and nematodes have only one HOX cluster. Of particular interest is that many of the HOX cluster genes function in the process of pattern formation along the anterior–posterior body axis. However, a lot of homeobox genes are not found in clusters and do not function in pattern formation, but play roles in the determination of cell fates and cell differentiation. Homeobox genes thus perform a plethora of roles for all aspects of the development of an organism.