Nick translation is a tagging technique where DNA polymerase I is used to replace some of the nucleotides of a DNA sequence with their labeled analogues. In this process, DNA molecules are first treated with DNase to produce single-stranded “nicks”. Then DNA polymerase I elongates the 3’ hydroxyl terminus of the nicked sites, removing nucleotides by 5’-3’ exonuclease activity and replacing them with modified dNTPs, thus labeling the DNA molecule. This technique is commonly used in fluorescence in situ hybridization (FISH) to label the probes.