Terminal transferase (TdT) is a template independent polymerase that catalyzes the addition of deoxyribonucleotide monophosphate from triphosphate to the 3′ hydroxyl terminus of DNA. Prefered substrates are: single-stranded DNA, double-stranded DNA with 3’-hydroxyl termini and oligodeoxynucleotide primers. Used for specific labeling of 3′-termini with ribonucleotides. Labels 3′-ends of DNA fragments with an [ α-³²P] 3′-deoxynucleoside. Adds homopolymer tails of deoxyribonucleotides to vectors or cDNAs.