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 (By similarity). In polyploid organisms, the Rad50/Mre11 complex appears to restrain the repair of double-strand breaks by homologous recombination, allowing another pathway to act as the primary mode of repair (PubMed:19593371). 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 are more resistant to DNA damage, but recover more slowly than the wild-type. Belongs to the MRE11/RAD32 family.