FEN-1 is a kind of structure-specsific nuclease, which could recognize specific DNA bifurcated structure and cleave single-strand DNA with free 5′ end. During DNA replication, FEN-1 5′→3′ exonuclease removes the last nucleoside of RNA primer attached to an Okazaki fragment. In DNA repair, FEN-1 5′→3′ endonuclease is involved in the process of removing the damaged nucleotide. FEN-1 gene contains two conserved regions and one PCNA binding region.