Terminal deoxynucleotidyl transferase (TdT) is a template-independent DNA polymerase that catalyzes the addition of deoxynucleotides to the 3' hydroxyl terminus of single or double stranded DNA molecules. The presence of 1 mM Co²⁺ stimulates the tailing of the 3'-ends of DNA fragments.

• Addition of homopolymer tails to the 3' ends of DNA
• Labeling the 3' ends of DNA with modified nucleotides (e.g., ddNTP, DIG-dUTP)
• TUNEL assay (in situ localization of apoptosis)
• TdT dependent PCR

1 unit is defined as the amount of polymerase required to convert 1 nmol of dATPs into acid insoluble material in 1 hour at 37°C.

• Terminal deoxynucleotidyl Transferase: 20,000 U/ml in 50 mM KPO₄, 100 mM NaCl, 1.0 mM DTT, 0.1 mM EDTA, 50% Glycerol, 0.1% Triton X-100, pH 7.3 @ 25°C
• 10X Reaction Buffer: 200 mM Tris-Acetate, 500 mM Potassium Acetate, 100 mM Magnesium Acetate, 10 mM DTT, pH 7.9 @ 25°C
• 2.5 mM CoCl₂