<p>The helix-loop-helix (HLH) DNA-binding domain consists of a closed bundleof four helices in a left-handed twist with two crossover connections. TheHLH domain directs dimerisation, and is juxtaposed to basic regions tocreate a DNA interaction interface surface that recognises specific DNAsequences. Basic region/HLH (bHLH) proteins regulate diverse biologicalpathways [<cite idref="PUB00014879"/>]. bHLH proteins include MyoD [<cite idref="PUB00014882"/>], SREBPs (sterol regulatory element binding proteins)[<cite idref="PUB00014881"/>], and yeast Pho4 (phosphatase system) [<cite idref="PUB00014880"/>].</p><p>In certain proteins the bHLH domain contains a leucine-zipper motif. ThebHLH/leucine zipper (bHLHZip) domain specifies dimerisation within a networkof proteins and determines sequence-specific DNA binding [<cite idref="PUB00014876"/>]. bHLHZip domains occur in the transcription factors Myc,Mad, Max and Usf [<cite idref="PUB00014877"/>, <cite idref="PUB00014878"/>]. </p><p>This entry is bHLHZip, which covers the bHLH domain and the leucine zipper motif, when present.</p>