Additional coimmunoprecipitations were carried out to directly address the requirement for the Mre11 C terminus. In this case, HA-tagged CDK2 was transiently expressed and precipitated from MEFs that expressed both wild-type endogenous Mre11 and either Mre11C54 or Mre11ATLD1 from cDNA (Fig. 3e). Whereas wild-type Mre11 was present in both coimmunoprecipitation eluates, Mre11 molecules with the large tag (Mre11C54) or C-terminal deletion (Mre11ATLD1) were absent. Thus, the in vivo association between CDK2–cyclin A and Mre11 requires an unperturbed Mre11 C terminus.