Terminal deoxynucleotidetransferase (TdT) is a nuclear polymerase with a molecular weight of about 58 kDa, which is normally expressed in normal thymic cortical cells and blast lymphocytes. TdT is widely used as a marker of lymphocytic leukemia and is strongly expressed in acute lymphoblastic lymphoma/leukemia, which is mainly used for the diagnosis of lymphoblastic lymphoma/leukemia.