Terminal Deoxynucleotidyl Transferase (TdT) is a specialized DNA polymerase that is primarily expressed in immature, pre-B, pre-T lymphoid cells, and acute lymphoblastic leukemia/lymphoma cells. TdT plays a role in catalyzing the addition of nucleotides to the 3' terminus of DNA molecules. Unlike most DNA polymerases, TdT does not require a template for its activity. It can add nucleotides to protruding 3' overhangs, as well as blunt or recessed 3' ends.

Normally, TdT is found in cortical thymocytes and primitive lymphocytes. The TdT antibody is used to detect the antigen of TdT, which is present in the nucleus of normal hematopoietic cells, normal cortical thymocytes, and in the cytoplasm of megakaryocytic in the bone marrow. TdT expression is observed in over 90% of cases of acute lymphocytic leukemia (except for pre-B-cell ALL) and is absent in normal mature T or B lymphocytes. In chronic myeloid leukemia, approximately one-third of cases show positive TdT expression, indicating a favorable response to chemotherapy.