Amphipathic α-helices create the dimerization interface in the bZIP and bHLH classes of DNA-binding proteins. These amphipathic helices have been shown to enter into a wide variety of specific dimerization interactions, and this large array of possible combinatorial interactions may provide for fine control of biological function. In bHLH-ZIP proteins, the addition of a leucine-zipper region immediately carboxyl-terminal to the helix-loop-helix region provides for an additional level of both dimerization specificity and control, again through the interaction of amphipathic α-helices. Interhelical electrostatic interactions have been implicated in regulating dimerization specificity.