The C terminus of Sir4 forms a coiled-coil structure. The coiled-coil domain is responsible for the dimerization of Sir4 and contains the binding site of Sir3. Structural and biochemical analyses of the Sir4 coiled-coil domain provide important insights into the molecular mechanisms of Sir3-Sir4 interaction and the assembly of a ternary Sir2/Sir3/Sir4 complex that are essential for epigenetic control of gene expression in S. cerevisiae.