One of the most puzzling problems in molecular biology is that of specific recognition of one macromolecule by another. Our entire immune system is based on our antibodies' ability to specifically recognize antigenes. The inter- and intracellular signal transduction is dependent upon specific recognition. The correct gene expression is impossible if a unique sequence cannot be recognized or if a wrong sequence is bound by a transcription factor. Of a particular importance and interest is to understand the mechanisms of the specific recognition of DNA by proteins.