Terminal Deoxynucleotidyl Transferase (TdT) catalyzes the incorporation of single deoxynucleotides into the 3'-OH terminus of single- or double-stranded DNA, making it an effective agent in 3' end labeling applications such as Terminal dUTP nick end labeling (TUNEL). Terminal Deoxynucleotidyl Transferase (TdT) requires an oligodeoxynucleotide of at least three bases as a primer, but is not template-dependent.