Exonuclease with a preference for double-stranded DNA with mismatched 3' termini. May play a role in DNA repair. Exonucleolytic cleavage in the 3'- to 5'-direction to yield nucleoside 5'-phosphates. Binds 2 Mg(2+) per subunit. The second magnesium ion interacts with only one residue. Substitution with Mn(2+) results in partial activity. Homodimer. Belongs to the exonuclease superfamily. TREX family.