The return of CtIP to measurable levels by proteasome inhibition permitted further characterization of the protein in the absence of the Mre11 C terminus. We used phage lambda phosphatase to analyze the phosphorylation status of CtIP in normally dividing cells. In con¬trol cells, most detectable CtIP was phosphorylated, and proteasome inhibition had no detectable effect (Fig. 2b). By contrast, CtIP was clearly hypophosphorylated in the absence of MRN (Mre11−/−) or of the Mre11 C terminus (Mre11ATLD1). Because CDK-dependent phosphorylation of CtIP is required for interaction with BRCA1 (refs. 19,20,24), we carried out coimmunoprecipitation experiments to assess CtIP-BRCA1 interaction in cells lacking the Mre11 C terminus. Indeed, BRCA1-CtIP interaction is disrupted when MRN is deficient (Mre11−/−), or in the context of Mre11ATLD1 (Fig. 2c).