Template-independent DNA polymerase which catalyzes the random addition of deoxynucleoside 5'-triphosphate to the 3'-end of a DNA initiator. 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. a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) Can also utilize other divalent cations, such as Mn(2+) and Co(2+) (in vitro). Found in the thymus and not in the spleen, kidney, intestine, or liver. During ontogeny TDT appears in significant amounts in the thymus of tadpoles at metamorphic climax but little in the earlier midlarval stages. Belongs to the DNA polymerase type-X family.