An account is given of the discovery of the classical Cys2His2 zinc finger, arising from the interpretation of biochemical studies on the interaction of the Xenopus protein transcription factor IIIA with 5S RNA, and of structural studies on its structure and its interaction with DNA. The finger is a self-contained domain stabilized by a zinc ion ligated to a pair of cysteines and a pair of histidines, and by an inner hydrophobic core. This discovery showed not only a new protein fold but also a novel principle of DNA recognition. Whereas other DNA binding proteins generally make use of the two-fold symmetry of the double helix, zinc fingers can be linked linearly in tandem to recognize nucleic acid sequences of varying lengths. This modular design offers a large number of combinatorial possibilities for the specific recognition of DNA (or RNA). It is therefore not surprising that the zinc finger is found widespread in nature, including 3% of the genes of the human genome. The zinc finger design is ideally suited for engineering proteins to target specific genes. In the first example of their application in 1994, a three-finger protein was constructed to block the expression of an oncogene transformed into a mouse cell line. In addition, a reporter gene was activated by targeting an inserted zinc finger promoter. Thus, by fusing zinc finger peptides to repression or activation domains, genes can be selectively switched off or on. It was also suggested that by combining zinc fingers with other effector domains, e.g., from nucleases or integrases, to form chimeric proteins, genomes could be manipulated or modified. Several applications of such engineered zinc finger proteins are described here, including some of therapeutic importance. Source.