The best-understood protein structure involved in DNA binding is the helix-turn-helix motif. A second DNA-binding domain, the finger structure, has been proposed on the basis of sequence analysis, partial proteolysis and zinc content of Xenopus transcription factor TFIIIA. Other eukaryotic proteins were subsequently found to contain contiguous repeat units of the postulated finger motif. Each repeat unit contains thirty amino acids and is thought to bind a zinc atom using two cysteines and two histidines as ligands. The protein loop or finger between apparent zinc ligands is rich in DNA-binding residues and is thought to make specific contacts with DNA. The yeast protein ADR1, a positive regulator of transcription of the gene ADH2, contains two finger domains in a region of the protein required for transcriptional activation. Nineteen independently isolated adr1 mutations induced by hydroxylamine were found at nine different amino-acid positions, seven of which are in the two finger domains. All four mutations that altered invariant cysteine or histidine residues led to an adr1 null phenotype. Only one other mutation caused an adr1 null phenotype. Thus, one finger domain is not sufficient for ADR1 activity. This provides the first evidence that, as is consistent with the proposed model, the invariant cysteine and histidine residues are essential for the formation of the finger structure.