The Helix-Turn-Helix motif consists of two α helices and a short extended amino acid chain between them. The more carboxyl-terminal helix can fit into the major groove of DNA. This motif is found in hundreds of DNA-binding proteins, including λ repressor, tryptophan repressor, catabolite activator protein (CAP), octamer transcription factor 1 (Oct-1) and heat shock factor (HSF),
Homeotic genes are known to play a critical role in fly development. Their products contain a homeodomain which is a special form of the helix-turn-helix motif.