Part of the Rad50/Mre11 complex, which is involved in the early steps of DNA double-strand break (DSB) repair. Mre11 binds to DSB ends and has both double-stranded 3'-5' exonuclease activity and single-stranded endonuclease activity. Binds 2 manganese ions per subunit. Nuclease activity is regulated by Rad50. Homodimer. Forms a heterotetramer composed of two Mre11 subunits and two Rad50 subunits. Mutants show a slight growth defect, but do not show increased sensitivity to ionizing radiation or alkylating agents. Show decreased rates of survival after UV-C irradiation. Display extensive delay in the repair of DNA double-strand breaks. Belongs to the MRE11/RAD32 family.