Transcription factors containing basic-helix-loop-helix (bHLH) domains interact to regulate differentiation in a number of cellular systems, including myogenesis, neurogenesis and haematopoiesis []. The DNA-binding activity of these proteins is mediated via the basic region of the bHLH domain, and is dependent upon formation of homo- and/or heterodimers by these transcription factors.The Id family of proteins (whose members include Id1, Id2, Id3 and the Drosophila homologue emc) contain a HLH-dimerization domain but lack the DNA-binding basic region. Formation of heterodimers between these proteins and bHLH domain-containing transcription factors abolishes the DNA-binding ability of the latter, thus inhibiting their activity [].
