['Poly purine.pyrimidine sequences have the potential to adopt intramolecular triplex structures and are overrepresented upstream of genes in eukaryotes. These sequences may regulate gene expression by modulating the interaction of transcription factors with DNA sequences upstream of genes.', 'A poly purine.pyrimidine sequence with the potential to adopt an intramolecular triplex DNA structure was designed. The sequence was inserted within a nucleosome positioned upstream of the beta-galactosidase gene in yeast, Saccharomyces cerevisiae, between the cycl promoter and gal 10 Upstream Activating Sequences (UASg). Upon derepression with galactose, beta-galactosidase gene expression is reduced 12-fold in cells carrying single copy poly purine.pyrimidine sequences. This reduction in expression is correlated with reduced transcription. Furthermore, we show that plasmids carrying a poly purine.pyrimidine sequence are not specifically lost from yeast cells.'] Do poly purine.pyrimidine sequences upstream of the beta-galactosidase gene affect gene expression in Saccharomyces cerevisiae?