Transferase that catalyzes the nontemplated addition of nucleoside triphosphate to coding ends during V(D)J recombination (N addition). Involved in the generation of diversity in the antigen-binding region of immunoglobulin heavy and light chains and T-cell receptors during B- and T-cell development. Does not act on double-stranded DNA with blunt ends. 3'-to-5' DNA exonuclease. Involved in the generation of diversity in the antigen-binding region of immunoglobulin heavy and light chains and T-cell receptors during B- and T-cell development. Acts on single-stranded and double-stranded DNA with 3' or 5' extensions, but not on double-stranded DNA with blunt ends. Attenuates not only isoform TDT-S-catalyzed N addition, but also P (palindromic) addition in coding joins (PubMed:11938351). Lacks terminal transferase activity (PubMed:11136823, PubMed:7556063). 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). In assays with isoform TDT-S. Interacts with PRP19 and DNTTIP1. Forms a ternary complex with DNTTIP2 and core histone. Released from this complex by PCNA. Interacts with TRERF1. The subcellular location is controversial. Detected in the nucleus (PubMed:11136823). Found mainly in the cytoplasm (PubMed:7556063). Isoform TDT-L: Expressed in the thymus, and, at lower levels, in the bone marrow (PubMed:8464703, PubMed:11136823, PubMed:7556063). Detected in both cycling and noncycling pro-B and pre-B cells (at protein level) (PubMed:11938351). Isoform TDT-S: Expressed in both cycling and noncycling pro-B, but not pre-B, cells (at protein level) (PubMed:11938351). Not detected in mature peripheral or germinal center B cells (PubMed:11938351). Major form in the thymus and the bone marrow (PubMed:8464703, PubMed:11136823). Catalyzes the nontemplated addition of nucleoside triphosphate to coding ends during V(D)J recombination (PubMed:23856622). May have a longer half-life than isoform TDT-L (PubMed:7556063). Exhibits 3'-to-5' DNA exonuclease activity (EC=3.1.11.-) (PubMed:23856622). May have a shorter half-life than isoform TDT-S (PubMed:7556063, PubMed:10878023). Belongs to the DNA polymerase type-X family.