The Klenow Fragment corresponds to the 70-kDa cleavage product of the E. coli DNA Polymerase I with the protease, subtilisin. It lacks the 5’-3’ exonuclease activity of DNA Pol I, a feature that makes the enzyme unsuitable in molecular biology purposes. The Klenow fragment, however, retains the original polymerase activity as well as the 3’-5’ proofreading activity. This enzyme is thus suitable for DNA synthesis applications such in probe generation, dideoxy sequencing, DNA blunting of overhangs, and DNA labeling, among others.