Terminal deoxynucleotidyl transferase (TdT) is a unique DNA polymerase present in normal and malignant pre-B and pre-T cells during early differentiation. TdT changes the addition of deoxynucleoside 5'-triphosphate to the 3'-end of a DNA initiator without template direction.  Tdt is present in the cells during immunoglobulin gene rearrangement and plays an important role in lymphocyte maturation. TdT contributes to the generation of junctional diversity in antigen receptors of immature lymphocytes. TdT is expressed in lymphoid precursors of B- and T-cell lineage in thymus and bone marrow. Anti-TdT stains nearly all acute lymphoblastic lymphoma/leukemia (ALL) cases, but does not stain pre-B-cell ALL or mature B- and T-cells. Anti-TdT staining is also useful for identifying Type AB thymoma and some chronic myeloid leukemia. It is a sensitive and specific marker for lymphoblastic lymphoma/leukemia.