Terminal deoxynucleotidyltransferase (TdT) is a DNA polymerase expressed in immature lymphocytes of the thymus and bone marrow, as well as certain leukemic cells. The nuclear enzyme TdT plays a critical role in generating immune receptor diversity. During rearrangement of Ig and TCR V region genes, TdT catalyzes the addition of N nucleotides without a DNA template to V-D and D-J junctions in both Ig and TCR as well as the V-J junctions in the TCR (1–3). During the fetal and newborn period, TdT activity is absent, resulting in an N region-deficient Ag receptor repertoire significantly lacking in the diversity observed in the junctional regions of Igs and TCRs of adult animals