Terminal deoxynucleotidyl transferase (TdT), an enzyme normally present in the human thymus and in a subpopulation of marrow cells, has the unique ability to catalyze the addition of deoxynucleoside triphosphates onto the single-strand, free end of DNA in a template-independent fashion.1 The presence of TdT within the lymphoblasts of a child with acute lymphoblastic leukemia was first noted 10 years ago.2 Since then, the detection of TdT has become a conventional laboratory aid to morphologic studies for the classification of leukemia, the enzyme being present in nearly every case of acute lymphoblastic leukemia examined.1,3,4 Several years ago, Donlon and co-workers . . .