Fig. 1. The functional domains of Myod. Myod (red) forms a heterodimer with an E-protein (green) through the helix-loop-helix domains (helix1 and helix2). The adjacent basic regions (also in an alpha helical conformation) contact the DNA. In Myod, the basic region also contains the `myogenic code'. This consists of three residues that are conserved in all of the myogenic bHLH proteins (Myod, Myf5, Myog and Mrf4), which do not directly affect DNA binding but are necessary to activate the transcription of specific muscle genes by either interacting with co-factors or inducing confomational change, or both. Myod has a single transcriptional activation domain (AD), and a histidine- and cysteine-rich (H/C) region that contains a tryptophan residue that is needed for Myod to interact with the Pbx/Meis complex. The helix 3 region is also required for Myod to cooperatively bind to the Pbx/Meis complex at the Myogenin (Myog) promoter. The E-protein has two independent activation domains (AD1 and AD2) and a domain that can repress the function of either activation domain (rep).