Helix-loop-helix DNA-binding domain. Illustration of a regulatory protein (blue) binding to DNA (deoxyribonucleic acid, orange helix) in a helix-turn-helix formation. Such protein-DNA binding can enhance or suppress expression of the genetic information carried by the DNA. In this type of DNA-binding protein, the short and long alpha-helices of the protein are joined by a 'loop' (centre left) and bound as a dimer to the DNA structure. The key protein component is called the 'recognition alpha helix'. For this artwork without labels, see C023/8883.