Although required, the nuclease activities of Mre11 alone are not sufficient for resection. This requires the BRCA1 tumor suppressor and the CtIP protein, both bound to MRN. CtIP shares a limited region of homology with the Sae2 nuclease in Saccharomyces
cerevisiae, but unlike Sae, CtIP has not yet been shown to possess nuclease activities. Rather, CtIP interaction appears to be required to enhance Mre11 nuclease activity. For efficient resection in vivo, CtIP must interact with BRCA1 (refs. 19,20). It is the formation and dissolution of the MRN–CtIP–BRCA1 complex that provides the regulation of resection capacity (and thus homologous recombination capacity) throughout the cell cycle19,20. This complex is only found in S and G2 phases