Sirtuin 1 (Sirt 1) is encoded by a gene that encodes a number of the sirtuin-like proteins, all homologs to the yeast Sir2 protein. Members of the sirtuin family are characterized by a sirtuin core domain. Yeast sirtuin proteins are known to regulate epigenetic gene silencing, suppress recombination of DNA and repressess rDNA transcription by the RNA pol. Studies suggest that the human sirtuins regulate apoptosis by impairing proapoptotic ability and modulating cell senescence. They regulate muscle differentiation by deacetylating key proteins. They also deacetylate the following: 'Lys-382' of p53/TP53, TAF1B, 'Lys-266' of SUV39H1 (leading to its activation), 'Lys-26' of HIST1H1E, APEX1 at 'Lys-6' and 'Lys-7' and 'Lys-16' of histone H4 (in vitro), as well as H2A. Sirtuins are also involved in HES1- and HEY2-mediated transcriptional repression and inhibit skeletal muscle differentiation by deacetylating PCAF and MYOD1. They may serve as a sensor of the cytosolic ratio of NAD+/NADH, which is essential in skeletal muscle cell differentiation. The eNoSC complex is able to sense the energy status of cell: upon glucose starvation, elevation of NAD+/NADP+ ratio activates SIRT1, leading to histone H3 deacetylation followed by dimethylation of H3 at 'Lys-9' (H3K9me2) by SUV39H1 and the formation of silent chromatin in the rDNA locus. In the case of HIV-1 infection, sirtuin interacts with and deacetylates the viral Tat protein. Cellular AP endonuclease activity is stimulated by promoting the association of APEX1 to XRCC1. Red wine, which contains resveratrol, may participate in activation of sirtuin proteins, and may therefore participate in an extended lifespan as observed in yeast.