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helix-turn-helix (helix-loop-helix) A structural motif characteristic of certain proteins that bind to DNA. It consists of two alpha helices connected by a short nonhelical segment called a ‘turn’. One of the helical segments recognizes a specific sequence of nucleotides in the DNA molecule and fits into the groove in the DNA double helix, while the other helix stabilizes the bound configuration. This motif is one of several found in DNA-binding proteins (see also leucine zipper; zinc finger).