To further examine the MRN-CtIP relationship, we used murine cells harboring loxP-flanked (floxed) conditional allele of Mre11 (Mre11cond/−) in which RAD50 and NBS1 protein levels are reduced when Mre11 is rendered deficient (Mre11−/−) (Supplementary Fig. 1). In MRN-deficient cells (Mre11−/−), CtIP protein levels were markedly reduced (Supplementary Fig. 2). By contrast, BRCA1 levels were unaffected. CtIP levels were also assessed in murine cells expressing Mre11H129N (Mre11H129N/−) (Supplementary Fig. 1), a targeted germline missense mutation causing defective Mre11 nuclease activities while preserving integrity of the MRN complex. No effect on CtIP levels was observed in these cells (Supplementary Fig. 2).