Understanding the mechanisms controlling transcription of a gene requires the identification and characterization of its -acting regulatory elements. A highly useful approach to the identification and characterization of -acting elements has been the systematic coupling of genomic fragments to reporter constructs, so called "promoter bashing". The expression from such reporters must be normalized for differences in transient transfection efficiency between cells and replicates. A novel dual color fluorescent reporter system to assay the promoter activity of a genomic DNA fragment of interest was established by cloning a red fluorescent protein gene and a green fluorescent protein gene into a single vector, giving a system in which the ratio between red and green fluorescence is proportional to promoter activity. This system allows real time quantitative monitoring of promoter activity. We validated this approach by assaying the -acting regulatory potential of the peroxisome proliferators-activated receptor gamma2 gene.