Sirtuin enzymes were discovered in yeast for their exciting ability to extend the lifespan of yeast. Biochemical work dissecting how these proteins affect longevity led to the discovery that this type of enzyme uses NAD to help keep certain genes in the cell “silent,” or non-functional. To do this, the sirtuin enzymes break NAD and attach a portion of it to proteins in the vicinity of target genes, and in doing so, “silences” them. https://www.nature.com/articles/35001622