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.
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Reactome:R-HSA-174441 "Removal of RNA primer and dissociation of RPA and Dna2 from the C-strand"; reactome:R-HSA-110363 "FEN1 bound to PCNA and APEX1 cleaves flap ssDNA"; reactome:R-HSA-174446 "Removal of remaining Flap from the C-strand"; reactome:R-HSA-5651782 "FEN1 bound to POLB cleaves displaced DNA strand (flap)"; reactome:R-HSA-5687664 "FEN1 cleaves displaced ssDNA flaps during MMEJ"; reactome:R-HSA-69152 "Removal of remaining Flap"