A basic helix–loop–helix (bHLH) is a protein structural motif that characterizes one of the largest families of dimerizing transcription factors.

In general, transcription factors (including this type) are dimeric, each with one helix containing basic amino acid residues that facilitate DNA binding.

bHLH proteins typically bind to a consensus sequence called an E-box, CANNTG.

These proteins contain an additional COE domain Since many bHLH transcription factors are heterodimeric,[8] their activity is often highly regulated by the dimerization of the subunits.

They are, however, able to form heterodimers with proteins that have the bHLH structure, and inactivate their abilities as transcription factors.