An essential part of gene expression and regulation is the binding of a regulatory protein to the recognition sequence of the gene on which it acts. Many such proteins have embedded in their structures a domain, or motif, that serves for binding to DNA. The bestunderstood protein structure used for DNA binding is the helix-turn-helix motif seen in the crystal structure of several bacterial regulatory proteins (for review, see Pabo and Sauer 1984; Anderson et al. 1987). Amino acid sequences that could form a similar structural motif are also present in the homeo box region of several eukaryotic proteins (Gehring 1985). A few years ago, it was discovered in this laboratory (Miller et al. 1985) that the Xenopus transcription factor IIIA (TFIIIA) for the 5S RNA gene contains small sequence units repeated in tandem, and it was proposed that each unit is folded about a Zn atom to form separate structural domains. Similar units have subsequently been found to be present in the amino acid sequence of other transcription factors and more generally nucleic-acid-binding proteins. Thus a second and apparently more commonly used structural motif for DNA recognition has emerged: the" Zn finger."