Helix-loop-helix motif:

The helix-loop-helix (HLH) dimerization domain is rather distinct from the helix- turn-helix motif described above that is included in DNA binding not dimerization and must not be confused with it. An HLH domain consists of two α-helices divided through a nonhelical loop. A C-terminal α-helix has hydrophobic amino acids on one face. Therefore two transcription factor monomers, every with an HLH motif, can dimerize through interaction among the hydrophobic faces of the two C-terminal α-helices. Such as the leucine  zipper,  the HLH  motif  is  frequently found in transcription factors which contain basic DNA binding  domains. Over like the leucine zipper the HLH motif can dimerize transcription factor monomers to form either heterodimers or homodimers. This ability to form heterodimers markedly raises the variety of active transcription factors which are possible and so increases the potential for gene regulation.