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. Optimum pH is 7.5-8.0. Homodimer. Detected in heart, breast, prostate, skeletal muscle, testis, uterus, bone marrow, colon, small intestine, stomach and thymus. Only supported by readthrough transcripts formed via the splicing of exons from UCHL5IP and TREX2 genes. Belongs to the exonuclease superfamily. TREX family.