An in vitro procedure used to radioactively label a DNA of interest uniformly to a high specific activity. First, nicks are introduced into the unlabeled DNA by an endonuclease, generating 3′ hydroxyl termini. E. coli DNA polymerase I is then used to add radioactive residues to the 3′ hydroxy terminus of the nick, with concomitant removal of the nucleotides from the 5′ side. The result is an identical DNA molecule with the nick displaced further along the duplex. See strand-specific hybridization probes.