T4 PNK can label nucleic acids with 5′-phosphate in the exchange reaction using reaction buffer B. However, for efficient labeling we recommend to remove the 5′-phosphate groups of DNA/RNA (e.g. by using Thermo Scientific FastAP, Cat. Nos. EF0651/2/4) and then phosphorylate in the forward reaction using T4 PNK with the buffer A.