DNA nucleotidylexotransferase (UniProt: P04053; also known as EC:2.7.7.31, Terminal addition enzyme, Terminal deoxynucleotidyltransferase, TdT, Terminal transferase) is encoded by the DNTT (also known as TDT) gene (Gene ID: 1791) in human. DNA polymerases are essential for replication, repair, and recombination of nucleic acid. In these processes, the polymerase extends a primer using a DNA template to guide each incorporation event. However, the requirement for using a template is not universal because cells possess terminal deoxynucleotidyl transferase (TdT) that has the ability to incorporate nucleotides in a template-independent manner using only single-stranded DNA as the nucleic acid substrate. TdT catalyzes the addition of deoxynucleotides to the 3′ hydroxy terminus of either double or single stranded DNA in a template-independent manner. Although TdT preferentially adds deoxynucleotides to 3′-extensions, tailing onto 5′ overhangs or blunt ended double strands of DNA can also be achieved. TdT is present in immature pre-B and pre-T cells where it inserts N-nucleotides to the V (D) J gene segment during rearrangements of genes. It plays a vital role in the development and variation of the immune system in vertebrates. TdT is commonly used for labeling DNA fragments and vectors with homopolymer tails and/or with modified deoxynucleotides, such as biotin-dNTPs, 32P-dNTPs, or ddNTP. Three alternative splice variants of TdT have been described in human. They are designated as TdTS (short), TdTL1 (long) and TdTL2 (long). TdTL1 and TdTL2 both localize in the nucleus, however, TdTL2 is expressed more abundantly in normal small lymphocytes compared to hTdTL1, which is readily detected in transformed lymphoid cell lines. Both long isoforms possess 3 →5 exonuclease activity for nucleotide removal whereas the short isoform performs nucleotide elongation of the coding ends during V(D)J recombination. Overexpression of TdTS or TdTL2 is reported to independently reduce the efficiency of V(D)J recombination, but the simultaneous overexpression of TdTS and TdTL2 can restore normal recombination frequencies. (Ref.: Gholami, S., et al. (2017). Adv Pharm Bull. 7(2); 215-220; Motea, EA., and Berdis, AJ. (2010). Biochim. Biophys. Acta. 804(5);1151-1166).，官网链接：https://www.sigmaaldrich.cn/product/mm/mabe2043 默克 科研、开发、生产。 作为生命科学行业的全球领先供应商，我们致力于为科研、生物技术开发和生产，以及制药药物疗法开发和生产提供各类解决方案和服务。

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