Terminal deoxynucleotidyl transferase (TdT) is a unique DNA polymerase that changes the addition of deoxynucleoside 5'-triphosphate to the 3'-end of a DNA initiator without template direction. 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. Foci of TdT positive cells maybe observed in peripheral lymphoid tissues. TdT is also present in malignant tumors of lymphoblastic lineage and thymoma. It is a sensitive and specific marker for lymphoblastic lymphoma/leukemia.