DNA Polymerase I, also known as Pol I, was initially isolated from E. coli and was the first polymerase to be extensively studied. Encoded by the gene polA, the 928-amino acid enzyme has several activities: A 5’-3’ polymerase activity, 5’-3’ exonuclease activity, and a 3’-5’ exonuclease activity. Commercial recombinant DNA polymerase I is usually expressed as a truncated enzyme that does not contain the 5’-3’ exonuclease activity. This version is called the Klenow fragment. DNA Pol I has found use in DNA modification applications such as in nick translation with DNase, and second-strand synthesis of cDNA.