sentences:  Organisms can have more than one FEN homologue; this redundancy may give an indication of the importance of these enzymes.In prokaryotes, the FEN enzyme is found as an N-terminal domain of DNA polymerase I, but some prokaryotes appear to encode a second homologue.The endonuclease activity of FENs was initially identified as acting on a DNA duplex which has a single-stranded 5' overhang on one of the strands (termed a "5' flap", hence the name flap endonuclease).