TdT also known as DNTT is a member of the DNA polymerase type-X family. TdT plays a crucial role in DNA synthesis by catalyzing the addition of deoxyribonucleotides to the 3′-hydroxyl end of DNA primers without the need for a template, which is essential for precise labeling of DNA′s 3′ termini. The human TdT gene is located on chromosome 10q23-q24 and encodes a 510 amino acid protein that is synthesized as a single-chain peptide. This enzyme exhibits a slight preference for incorporating deoxyribonucleotides over ribonucleotides, which is significant for maintaining DNA strand integrity during replication and repair processes. TdT is predominantly found in immature thymocytes, certain bone marrow cells, transformed pre-B and pre-T cell lines, as well as in leukemia cells, highlighting its importance in both normal and pathological cellular contexts.