Anti-TdT antibody labels normal cortical thymocytes and primitive lymphocytes. Anti-TdT antibody detects an enzyme found in the nucleus of normal hematopoietic cells, normal cortical thymocytes and in the cytoplasm of megakaryocytes of the bone marrow. TdT expression is seen in over 90% of acute lymphoblastic lymphoma/ leukemia cases with the exception of pre-B-Cell ALL. TdT expression is not seen in normal mature T-or B-lymphocytes. Anti-TdT is positive for approximately one third of all cases of chronic myeloid leukemia, making it a good indicator of better response to chemotherapy.