Terminal deoxynucleotidyl transferase (TdT) is an intracellular protein characteristic of certain primitive lymphocytes in normal thymus and bone marrow. It can be detected by enzyme assay or immunocytochemical staining. High levels of enzyme and increased numbers of TdT+ cells are found in some lymphoblastic leukemias and lymphomas. Analysis for TdT thus provides a useful adjunct in the differential diagnosis of leukemia.

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