DNA endonuclease activity, producing 5'-phosphomonoesters 5'-flap endonuclease activity 5' flap endonuclease activity Reactome:R-HSA-110363 Reactome:R-HSA-174446 Reactome:R-HSA-5687664 Reactome:R-HSA-69152 Reactome:R-HSA-174441 GO:0017108 Reactome:R-HSA-9914870 Reactome:R-HSA-5651782 Catalysis of the cleavage of a 5' flap structure in DNA, but not other DNA structures; processes the 5' ends of Okazaki fragments in lagging strand DNA synthesis. molecular_function flap endonuclease activity