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]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 8, Manipulating Proteins, DNA, and RNA: DNA polymerases and end labelling
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