MyoD is a basic helix loop helix (bHLH) myoblast specific transcription factor defined as a 'master switch' gene in that it can convert other cell types into muscles if the gene is active in them. bHLH proteins Myf5, Myogenin and MRF4/Myf6 are highly related to MyoD and these along with MyoD form the 'MyoD family' of transcription factors, also called the myogenic regulatory factors (MRFs).
MRFs form transcriptionally active heterodimers with the widely expressed E proteins, a distinct group of bHLH proteins including E12/E47, ITF-2 and HEB. Dimerization of these proteins juxtaposes their basic domains forming a functional DNA binding domain. MyoD/E protein heterodimers preferentially bind the DNA consensus sequence referred to as an E-box (CANNTG) in the control regions of muscle-specific genes and activate gene transcription of genes that are expressed in skeletal muscle.