1.1. Discovery of Sirtuins The founding member of sirtuin is the yeast-silencing information regulator 2 (SIR2) protein, one of four proteins (SIR1− 4) required for silencing the mating-type information loci in yeast. 1 SIR2− 4, but not SIR1, are also required for gene silencing at telomeres. 2 SIR2 also mediates gene silencing at the rDNA (rDNA) loci, which was shown to be independent of other SIR proteins. 3, 4 Immunofluorescence imaging showed that SIR2 is mainly in the nucleolus and telomeres in yeast. 5 It was demonstrated that the silenced genetic loci have low histone acetylation levels compared to loci that are not silenced. Mutation in Sir2− 4 increased histone acetylation levels and overexpression of SIR2 but not other SIR proteins led to decreases in histone H4, H2B, and H3 acetylation. 6 Analyzing the sensitivity of yeast chromatin to micrococcal nuclease and dam methyltransferase indicated that Sir2 mutation affects the chromatin structure in the rDNA and mating-type loci. 7 These interesting discoveries on SIR2 were further elevated by two discoveries made by Guarente and co-workers. The first one was that SIR2 is important for the replicative life span of yeast cells, 8 a finding that was later extended to higher eukaryotic species, 9, 10 although the role of sirtuins in life span is highly controversial 11 and may depend on genetic background and diet conditions. 12, 13 The second discovery was that SIR2 is an NAD-dependent histone deacetylase (Figure 1), 14 which established SIR2 as a mechanistically novel lysine deacetylase