The fragment of Escherichia coli DNA polymerase I derived by removal of the 5′–3′ exonuclease domain. It has the function of adding bases to the unpaired stretch of DNA at the end of a growing DNA duplex.

The large fragment of E. coli DNA Polymerase I. The Klenow fragment retains DNA polymerase activity and 3'-5' exonuclease activity, and is used for DNA sequencing.

The Escherichia coli DNA polymerase, without the exonuclease subunit.

A product of proteolytic digestion of the DNA polymerase I from E. coli; it has both polymerase and 3´-exonuclease activities but not 5´-exonuclease activity. A dissociation constant that characterizes the binding of an enzyme to a substrate. The smaller the value of K, the tighter the binding of the enzyme to the substrate. Also called the Michaelis constant.

The Klenow fragment is a large protein fragment which is produced when DNA polymerase I from E. coli is cleaved by the protease enzyme subtilisin. It exhibits the 5’ → 3’ polymerase activity and the 3’ → 5’ exonuclease activity for removal of precoding nucleotides, but does not retain the 5' → 3' exonuclease activity.