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The Klenow Fragment, is a large fragment of E.coli. DNA polymerase I. It retains the 3'→5' exonuclease activity of DNA polymerase I, but lacks the 5'→3' exonuclease activity of the intact DNA polymerase I. The 3'→5' exonuclease activity of Klenow Fragment ensures accurate proofreading when synthesizing DNA. It is used to fill in the 5'overhang ends of double-stranded DNA; and double-stranded DNA 3'overhang flattening （also called trimming）. It can also be used for the synthesis of the second strand of cDNA or the synthesis of the second strand of site-specific mutation reaction.