Max is a nuclear localized bHLH-Zip protein that forms homodimers or heterodimers with Myc family members, including Myc, Mad1, Mad3, Mad4, Mxi1 and Mnt (or Rox). These dimers bind to the E-box sequence CACGTG in order to regulate cell growth, proliferation and apoptosis. Mlx (Max-like protein X) is a bHLH-Zip protein that is structurally and functionally related to Max. Like Max, Mlx is broadly expressed in many tissues and has a long half-life. Mlx also forms homodimers or heterodimers with members of the Myc family, specifically Mad1, Mad4 and Rox, and members of the Mondo family, to repress or activate transcription from CACGTG E-boxes. MondoA forms weak homodimers and preferentially forms heterodimers with Mlx. The MondoA/Mlx complex is primarily localized to the cytoplasm, but will translocate to the nucleus in response to leptomycin B. Mlx can also dimerize with WBSCR14, a protein involved in Williams-Beuren syndrome (WBS), to repress E-box transcription, which provides further evidence that Mlx is a critical element in a transcription factor network.