Terminal deoxynucleotidyl transferase (TdT) is a unique DNA polymerase that plays a crucial role in the immune system and DNA repair. Unlike most DNA polymerases, TdT can add nucleotides to the 3鈥?hydroxyl end of a DNA strand in a template-independent manner. This enzyme is primarily expressed in immature B and T lymphocytes and is essential for V(D)J recombination, a process that generates the vast diversity of antigen receptors in the adaptive immune system. By adding non-templated nucleotides (N-regions) at the junctions between V, D, and J gene segments, TdT significantly increases the diversity of T-cell receptors (TCRs) and B-cell receptors (BCRs), enhancing the immune system's ability to recognize a wide range of antigens. Additionally, TdT is involved in DNA repair, particularly in the non-homologous end joining (NHEJ) pathway, where it helps repair DNA double-strand breaks. Its ability to function without a template also makes TdT a valuable tool in biotechnology and genetic research, such as in DNA sequencing and end labeling techniques.