5'->3' double-stranded DNA exonuclease which may also possess a cryptic 3'->5' double-stranded DNA exonuclease activity. Functions in DNA mismatch repair (MMR) to excise mismatch-containing DNA tracts directed by strand breaks located either 5' or 3' to the mismatch. Also exhibits endonuclease activity against 5'-overhanging flap structures similar to those generated by displacement synthesis when DNA polymerase encounters the 5'-end of a downstream Okazaki fragment. Required for somatic hypermutation (SHM) and class switch recombination (CSR) of immunoglobulin genes. Essential for male and female meiosis. {ECO:0000269|PubMed:10364235, ECO:0000269|PubMed:10608837, ECO:0000269|PubMed:11809771, ECO:0000269|PubMed:11842105, ECO:0000269|PubMed:12414623, ECO:0000269|PubMed:12704184, ECO:0000269|PubMed:14636568, ECO:0000269|PubMed:14676842, ECO:0000269|PubMed:15225546, ECO:0000269|PubMed:15886194, ECO:0000269|PubMed:16143102, ECO:0000269|PubMed:9685493}. This is the function of ENSG00000174371 (EXO1, exonuclease 1).