A nucleotide sequence containing about 180 base pairs, which are identical or very similar in many eukaryotic organisms, that encodes a 60-amino acid sequence known as a homeodomain. Present in many eukaryotic regulatory proteins, this domain is an important region involved in the binding of transcription factors and other regulatory proteins to the DNA molecule. Homeodomain-containing proteins control the expression of many genes, particularly ones that direct the formation of body structures during embryonic development. A homeobox was first identified in the homeotic genes of Drosophila and has since been found in the homeotic genes of many animals (including humans) and in plants.