A set of nuclear proteins in SACCHAROMYCES CEREVISIAE that are required for the transcriptional repression of the silent mating type loci. They mediate the formation of silenced CHROMATIN and repress both transcription and recombination at other loci as well. They are comprised of 4 non-homologous, interacting proteins, Sir1p, Sir2p, Sir3p, and Sir4p. Sir2p, an NAD-dependent HISTONE DEACETYLASE, is the founding member of the family of SIRTUINS.