TdT (EP266)

TdT (terminal deoxynucleotidyl transferase) gene is a member of the DNA polymerase type-X family and encodes a template-independent DNA polymerase that catalyzes the addition of deoxynucleotides to the 3’-hydroxyl terminus of oligonucleotide primers. TdT protein is expressed in a restricted population of normal and malignant pre-B and pre-T lymphocytes during early differentiation, where it generates antigen receptor diversity by synthesizing non-germ line elements (N-regions) at the junctions of rearranged Ig heavy chain and T-cell receptor gene segments.1,2 TdT expression occurs in over 90% of acute lymphoblastic lymphoma/leukemia cases with the exception of pre-B-cell ALL. TdT expression does not occur in normal mature T-or B-lymphocytes. Therefore, anti-TdT is a useful tool in diagnosis of T-ALL/T-lymphoblastic lymphoma and B-ALL/B-lymphoblastic lymphoma.3-7 Anti-TdT is also a useful marker for Type AB thymoma8 and it is positive for approximately one-third of all cases of chronic myeloid leukemia.9