PAR and C/EBP family proteins are liver-enriched basic leucine zipper (bZip) transcription factors that bind to similar sites on the promoters of albumin and cholesterol 7 hydroxylase genes. However, C/EBP proteins have a more relaxed binding specificity than PAR proteins, in that they recognize many sites within promoter or randomly selected rat genomic DNA sequences that are ignored by PAR proteins. Thus, DNAse I protection experiments suggest that C/EBP recognizes a binding site with an affinity similar to the one of the cholesterol 7 hydroxylase gene promoter every 200 to 300 bp. The frequency of PAR protein binding sites with comparable affinities is about 20- fold lower in the rat genome. By using a PCR-based amplification assay we selected high affinity DMAbinding sites for C/EBPß and the PAR protein DBP from a pool of oligonucleotides. Both proteins indeed recognize similar sequences with the optimal core binding sequence 5'RTTAY.GTAAY3'. However, as expected, DBP, is considerably less tolerant to deviations from the consensus site. Here we have characterized a single amino acid substitution mutant of C/EBPß that increases its target site specificity. This protein, C/EBPßv>A, contains a valine to alanine substitution at position 13 of the basic domain (residue 216 of C/EBPß). C/EBPßV>A selectively binds only the subset of C/EBP sites that are also DBP sites, both as oligonucleotides and within the natural contexts of the albumin and cholesterol hydroxylase promoters.