two well-conserved CDK phosphorylation sites: Ser327, which is phosphorylated as cells enter S phase and is necessary for interaction with BRCA1 (refs. 19,20,24), and Thr847, which is located in the Sae2 homology region and is necessary for localization of CtIP to DSBs Phosphomimetic modification of the Thr847 site leads to hyperactivation of resection and increased genome instability. In mammals, the primary active CDK in S phase is CDK2 bound to cyclin A26. Consistent with a role in controlling CtIP, deficiency of CDK2 in mammalian cells has been shown to confer a defect in homologous recombination