Terminal Deoxynucleotidyl Transferase （TdT） is a DNA polymerase present in normal and malignant pre-B- and pre-T-cells during early differentiation. Anti-TdT stains nearly all acute lymphoblastic lymphoma/leukemia （ALL） cases, but does not stain pre-B-cell ALL or mature B- and T-cells. Anti-TdT staining is also useful for identifying Type AB thymoma and some chronic myeloid leukemia.
