Helix-turn-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 alpha-helices of the protein are aligned around a turning point ('turn', upper right) and occupying the major groove in the DNA structure. The key protein component is called the 'recognition alpha helix'. For this artwork without labels, see C023/8881.