Nick translation was the first method devised for the in vitro labeling of DNA (1). During the reaction the DNA to be labeled is nicked by DNase I yielding a free 3′ hydroxyl end. DNA polymerase I then adds a new nucleotide to this end. The 5′–3′ exonuclease activity of the polymerase then moves the “nick” along the strand in the 3′ direction. The addition of a radioactively labeled nucleotide to the reaction results in probes that can be used in hybridization reactions to DNA immobilized on Southern blots, colony lifts, etc.