The 827 amino acid yeast RAP1 protein interacts with DNA to regulate gene expression at numerous unrelated loci in the yeast genome. By a combination of amino, carboxy and internal deletions, we have defined an internal 235 amino acid fragment of the yeast RAP1 protein that can bind efficiently to the RAP1 binding site of the PGK Upstream Activation Sequence (UAS). This domain spans residues 361 to 596 of the full length protein and lacks any homology to the DNA binding 'zinc finger' or 'helix-turn-helix' structural motifs. All the RAP1 binding sites we have tested bind domain 361-596, arguing that RAP1 binds all its chromosomal sites via this domain. The domain could not be further reduced in size suggesting that it represents the minimal functional DNA binding domain. The relevance of potential regions of secondary structure within the minimal binding domain is discussed.