Representative structures of bHLH proteins from the Protein Data Bank . In each diagram, the protein is shown as a secondary-structure cartoon and the DNA double helix is shown in stick representation. (a) MyoD bHLH-domain homodimer (PDB code 1mdy). (b) Pho4 bHLH-domain homodimer (1am9). (c) SREBP-1a bHLH-domain homodimer (1aoaC). (d) Max-Mad heterodimer (1nlw). (e) Max-Myc heterodimer (1nkp). (f) Max-Myc heterotetramer (1nkp). In (d-f) the Max HLH monomer is shown in dark gray. The scales are not comparable between different structures.