Sirtuin 1 is a member of the sirtuin family of proteins, homologs of the Sir2 gene in S. cerevisiae. Members of the sirtuin family are characterized by a sirtuin core domain and grouped into four classes. The functions of human sirtuins have not yet been determined; however, yeast sirtuin proteins are known to regulate epigenetic gene silencing and suppress recombination of rDNA. Studies suggest that the human sirtuins may function as intracellular regulatory proteins with mono-ADP-ribosyltransferase activity. The protein encoded by this gene is included in class I of the sirtuin family. Sirtuin 1 is downregulated in cells that have high insulin resistance and inducing its expression increases insulin sensitivity, suggesting the molecule is associated with improving insulin sensitivity. Furthermore, SIRT1 was shown to de-acetylate and affect the activity of both members of the PGC1-alpha/ERR-alpha complex, which are essential metabolic regulatory transcription factors. In mammals, SIRT1 has been shown to deacetylate and thereby deactivate the p53 protein. SIRT1 also stimulates autophagy by preventing acetylation of proteins (via deacetylation) required for autophagy as demonstrated in cultured cells and embryonic and neonatal tissues. This function provides a link between sirtuin expression and the cellular response to limited nutrients due to caloric restriction. SIRT1 plays a role in activating T helper 17 cells, which contribute to autoimmune disease; efforts to activate SIRT1 therapeutically may trigger or exacerbate autoimmune disease. SIRT1, along with HDAC1 and the AP-1 promoter complex within D1-type dopaminergic medium spiny neurons, appears to be closely involved in the pathogenesis of addiction.