Klenow Fragment. Large fragment of E. coli DNA Polymerase l enzyme which retains both the polymerase and the proofreading 3’→5′ exonuclease activities of Polymerase l. Lacks the 5′-exonuclease activity. Used for Sanger dideoxy sequencing. Suitable for second strand cDNA synthesis. Used for 3′-end labeling and filling in 5′-protruding sticky ends.