Terminal deoxynucleotidyl transferase (TdT) is a highly conserved vertebrate enzyme that possesses the unique ability to catalyze the random addition of deoxynucleoside 5'-triphosphates onto the 3'-hydroxyl group of a single-stranded DNA. It plays an important role in the generation of immunoglobin and T-cell receptor diversity. One of the in vivo functions of this enzyme is the addition of nucleotides at the junction (N region) of rearranged Ig heavy chain and T-cell receptor gene segments during the maturation of B- and T-cells.