sentences:  FENs catalyse hydrolytic cleavage of the phosphodiester bond at the junction of single- and double-stranded DNA.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.