What are terminal transferases?

Terminal transferases (TdT) are specialized DNA polymerase enzymes expressed in immature, pre-B, pre-T lymphoid cells, and acute lymphoblastic leukemia/lymphoma cells. They catalyze the addition of single deoxynucleotides into the 3'-OH terminus of DNA molecules. Blunt-ended, protruding or recessed single-stranded or double-stranded DNA molecules serve as a substrate for terminal transferases. Terminal transferases are template independent polymerases. They require an oligodeoxynucleotide of minimum 3 bases as a primer. Terminal transferases are useful for labeling the 3′ terminus of DNA with modified nucleotides such as DIG-dUTP and ddNTP.