Terminal deoxynucleotidyl Transferase (TdT), also known as DNA nucleotidylexotransferase (DNTT) or terminal transferase, is a nuclear enzyme, molecular weight of 58 kDa, responsible for the template-independent addition of N-nucleotides at gene segment junctions of developing lymphocytes. TdT plays a critical role during TCR and Ig gene rearrangement and is expressed in immature, pre-B, pre-T lymphoid cells, and acute lymphoblastic leukemia/lymphoma cells, but not in mature lymphocytes or non-lymphoid cells. This 19-3, a mouse monoclonal antibody, recognizes human and mouse TdT.