In mammalian cells, DNA is often subjected to stresses such as ionizing radiation (IR) that can induce DNA double strand breaks (DSBs). In response to DNA DSBs, mammalian cells activate a rapid phosphorylation signaling cascade through the protein kinases, Ataxia-Telangiectasia Mutated (ATM) and ATM- and Rad3-Related (ATR). Many well-characterized DNA repair factors are phosphorylated by ATM in response to DSBs, and the sequential phosphorylation of some of these factors, including NBS1, delay cell cycle progression (checkpoint arrest) to allow time for DNA damage repair. Results from a new study suggest that phosphorylation of NBS1 is regulated by the acetylation status of the protein, which is modulated by SIRT1 deacetylase.