sentences:  Flap endonuclease
  
  Flap endonucleases (FENs, also known as 5' nucleases in older references) are a class of nucleolytic enzymes that act as both 5'-3' exonucleases and structure-specific endonucleases on specialised DNA structures that occur during the biological processes of DNA replication, DNA repair, and DNA recombination.Flap endonucleases have been identified in eukaryotes, prokaryotes, archaea, and some viruses.