Study of proteins that recognize specific DNA sequences has yielded much information, but the field is still in its infancy. Already two major structural motifs have been discovered, the helix-turn-helix and zinc finger, and numerous examples of DNA-binding proteins containing either of them are known. The restriction enzyme Eco RI uses yet a different motif. Additional motifs are likely to be found as well. There is a growing understanding of some of the physical chemistry involved in protein-DNA binding, but much remains to be learned before it becomes possible to engineer a protein that binds to a specific DNA sequence.