DNA Polymerase I, Large (Klenow) Fragment (about 68 kD) is a proteolytic product of E. coli DNA Polymerase I which retains polymerization and 3to5 exonuclease activity, but has lost 5to3 exonuclease activity. Klenow retains the polymerization fidelity of the holoenzyme without degrading 5 termini. It is applicable to DNA sequencing by the Sanger dideoxy method, fill-in of 5 overhangs to form blunt ends, removal of 3 overhangs to form blunt ends, second strand cDNA synthesis and second strand synthesis in mutagenesis protocols.