Klenow fragment

The Klenow fragment is a large fragment of Escherichia coli DNA polymerase I that retains DNA polymerization and proofreading exonuclease activities while lacking the intact enzyme’s 5′-to-3′ exonuclease domain. This separation made it useful for extending DNA ends without all the activities of the full enzyme. Engineered exonuclease-deficient variants are different reagents, so a statement about retained proofreading should specify the form being discussed. ^[ref]