The basic side chains of lysine residues often play essential roles in DNA-protein recognition. They are able to contribute to the overall affinity of an interaction through nonspecific charge-charge interactions with the phosphate backbone and contribute substantially to the specificity of the interaction by forming direct hydrogen bonds with functional groups on the edges of the bases. This dual role and their almost ubiquitous presence in the interface of DNA-protein complexes make them very attractive targets for chemical modification experiments.