Flap endonucleases have been identified in eukaryotes, prokaryotes, archaea, and some viruses.
Organisms can have more than one FEN homologue; this redundancy may give an indication of the importance of these enzymes.
FENs catalyse hydrolytic cleavage of the phosphodiester bond at the junction of single- and double-stranded DNA.
[6] Some FENs can also act as 5'-3' exonucleases on the 5' terminus of the flap strand and on 'nicked' DNA substrates.
Protein structure models based on X-ray crystallography data suggest that FENs have a flexible arch created by two α-helices through which the single 5' strand of the 5' flap structure can thread.