T7 DNA Polymerase was originally isolated from the T7 Bacteriophage. Commercially, it is produced in E. coli consisting of two subunits: the 84-kDa T7 gene 5 polymerase and the E. coli thioredoxin cofactor that is necessary for polymerase processivity. Similar to the E. coli DNA Polymerase I, it has a 5’-3’ polymerase activity active at 37°C. It features a robust 3’-5’ exonuclease activity on both single and double stranded DNA that can be modified by the presence of EDTA. T7 DNA polymerase is recommended for gap filling and site-specific mutagenesis.