The basic helix-loop-helix motif. This ribbon model shows the human homodimeric transcription factor Max bound to its DNA target site. The two amphipathic α helices of each Max subunit are shown in red and white; the loop is yellow in both. The two subunits form a four-helix bundle through association of their dimerizing α helices (white), and the DNA-binding α helices (red) extend from the bundle.