Terminal deoxynucleotidyl transferase (TdT), also known as DNA nucleotidylexotransferase (DNTT) or terminal transferase, is a member of the DNA polymerase type-X family and encodes a template-independent DNA polymerase that catalyzes the addition of deoxynucleotides to the 3'-hydroxyl terminus of oligonucleotide primers. TdT is a specialized DNA polymerase expressed early in pre-B and pre-T lymphoid cell development, and in acute lymphoblastic leukemia/lymphoma cells. TdT contributes to the generation of junctional diversity by synthesizing non-germ line elements (N-regions) at the junctions of rearranged Ig heavy chain and T cell receptor gene segments. The L10-2 clone has been shown to be useful for western blotting of human and mouse TdT.