DNA double-strand break repair by homologous recombination is initiated by DNA end resection, which is commenced by the Mre11–Rad50–Xrs2 complex and Sae2 in yeast. Here we report that the nonhomologous end joining factor Ku limits the exonuclease activity of Mre11 and promotes its endonuclease to cleave 5'-terminated DNA strands at break sites. Following initial endonucleolytic cleavage...