In the May 1 issue of G&D, Dr. Leonard Guarente (MIT) and colleagues provide the first functional characterization of a mammalian sirtuin protein other than SIRT1. The Sir2 enzyme family, called sirtuins, is involved in a wide variety of biological processes, and are conserved from bacteria to humans. The human genome encodes seven Sir2 homologs, termed SIRT1-7. Since their discovery in 1999, most research had focused on the role(s) of SIRT1, while the other members of this family have gone largely uncharacterized. Now, Dr. Guarente and colleagues reveal that SIRT7 is the only one of the seven sirtuins to preferentially localize to nucleoli, where it promotes RNA Polymerase I-mediated transcription of ribosomal DNA. Dr. Guarente emphasizes that "It is very interesting that Sirt7 appears to be the only mammalian sirtuin with a nucleolar function, given that one important function of the yeast sir2 in determining mother cell lifespan occurs in the nucleolus."