In the past five years there has been a great expansion in our knowledge of the role of zinc in the structure and function of proteins. Not only is zinc required for essential catalytic functions in enzymes (more than 300 are known at present), but also it stabilizes and even induces the folding of protein subdomains. The latter functions have been most dramatically illustrated by the discovery of the essential role of zinc in the folding of the DNA-binding domains of eukaryotic transcription factors, including the zinc finger transcription factors, the large family of hormone receptor proteins, and the zinc cluster transcription factors from yeasts. Similar functions are highly probable for the zinc found in the RNA polymerases and the zinc-containing accessory proteins involved in nucleic acid replication. The rapid increase in the number and nature of the proteins in which zinc functions is not unexpected since zinc is the second most abundant trace metal found in eukaryotic organisms, second only to iron. If one subtracts the amount of iron found in hemoglobin, zinc becomes the most abundant trace metal found in the human body.