The helix-turn-helix is a common structural motif in proteins, particularly in DNA-binding proteins. It consists of two alpha helices connected by a short turn, allowing the protein to fit into the major groove of DNA. This structure is crucial for the regulation of gene expression. This motif is often found in transcription factors, which are proteins that help control the transcription of specific genes. The homeodomain is a well-known example of a helix-turn-helix protein, playing a vital role in the development of organisms by regulating gene activity during embryonic development.