The SID domain of Saccharomyces cerevisiae silent information regulator 4, a Sir2p interaction domain; and related domainsSaccharomyces cerevisiae Sir2p, Sir3p, and Sir4p form a heterotrimeric complex which binds chromatin and represses transcription at the homothallic mating type (HM) loci and at subtelomeric regions. This domain model spans residues 742-893 of Sir4p. Sir4p forms a stable heterodimer with Sir2p, mediated by Sir4p residues included in this domain, and a pocket between Sir2p's catalytic domain and its non-conserved N-terminus. Sir4p also interacts with an array of additional factors, including Yku80p, a subunit of the telomeric Ku complex (Yku70p-Yku80p), which binds two sites within Sir4p, one at the N-terminus and one in the C-terminal residues, 731-1358. Other interaction factors include Esc1p (Establishes silent chromatin 1) which binds the Sir4p PAD domain (partitioning and anchoring domain, residues 950-1262), and Sir3p, Yku70p, and Rap1p (Repressor Activator Protein) which bind in its C-terminal coiled-coil (residues 1257-1358). Other Sir4p interacting factors include the Ty5 retrotransposon. Additional roles for Sir4p include roles in DNA repair, and in aging. A SIR4 mutant having a truncated Sir4p lacking a C-terminal coiled-coil domain, has an extended mean life span; deletion of the SIR4 gene leads to a decreased mean life span.