The proteins SIR1 through SIR4 are required for transcriptional repression of the silent mating type loci, HML and HMR. The proteins SIR2 through SIR4 repress mulitple loci by modulating chromatin structure. Involves the compaction of chromatin fiber into a more condensed form. Homodimer and interacts with SIR4 and RAP1 C-terminus. Interacts with MCM10. N-terminal acetylation by NatA is important for transcriptional silencing activity. Present with 1400 molecules/cell in log phase SD medium.