TdT is a specialized DNA polymerase that adds N-nucleotides to the V, D, and J exons during antibody gene recombination enabling junctional diversity. It is expressed in immature, pre-B, pre-T lymphoid cells, and acute lymphoblastic leukemia/lymphoma cells, it is not expressed in pre-B-Cell ALL and normal mature T-or B-lymphocytes. This antibody detects normal cortical thymocytes and primitive lymphocytes; it detects an enzyme found in the nucleus of normal hematopoietic cells, normal cortical thymocytes and in the cytoplasm of megakaryocytes of the bone marrow. 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.