5-3 exonuclease (IPR002421), acting as a flap endonuclease (FEN), catalyzes the exonucleolytic hydrolysis of blunt-ended double-stranded DNA substrates and the endonucleolytic cleavage of 5'-forked nucleic acids at the junctions formed between single-stranded double-stranded DNA. The basic FEN reaction can be done by the N-terminal 5'-3' exonuclease domain of DNA polymerase I.