Homeobox genes are a large family of similar genes that direct the formation of many body structures during early embryonic development. In humans, the homeobox gene family contains an estimated 235 functional genes and 65 pseudogenes (structurally similar genes that do not provide instructions for making proteins). Homeobox genes are present on every human chromosome, and they often appear in clusters. Many classes and subfamilies of homeobox genes have been described, although these groupings are used inconsistently.
Homeobox genes contain a particular DNA sequence that provides instructions for making a string of 60 protein building blocks (amino acids) known as the homeodomain. Most homeodomain-containing proteins act as transcription factors, which means they bind to and control the activity of other genes. The homeodomain is the part of the protein that attaches (binds) to specific regulatory regions of the target genes.
Genes in the homeobox family are involved in a wide range of critical activities during development. These activities include directing the formation of limbs and organs along the anterior-posterior axis (the imaginary line that runs from head to tail in animals) and regulating the process by which cells mature to carry out specific functions (differentiation). Some homeobox genes act as tumor suppressors, which means they help prevent cells from growing and dividing too rapidly or in an uncontrolled way.
Because homeobox genes have so many important functions, mutations in these genes are responsible for a variety of developmental disorders. For example, mutations in the HOX group of homeobox genes typically cause limb malformations. Changes in PAX homeobox genes often result in eye disorders, and changes in MSX homeobox genes cause abnormal head, face, and tooth development. Additionally, increased or decreased activity of certain homeobox genes has been associated with several forms of cancer later in life.
The HUGO Gene Nomenclature Committee (HGNC) provides an index of gene families and their member genes.
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