The PAH2 domain of mSin3A adopts a left-handed, up-and-down, four-helix bundle structure with residues in all four helices as well as in the turn regions defining a compact structural domain with an extensive hydrophobic core. Helices α1 and α2 form a deep hydrophobic pocket, which constitutes the primary interaction surface for the Mad1 (6-21) peptide. The Mad1 (6-21) forms an amphipathic α helix in the complex and interacts with PAH2 mainly through the apolar surface of the helix[1].