Reporter vectors are very often used to investigate mechanisms responsible for regulation of promoter activity. Since their first generation, many new variants were constructed to increase sensitivity and reduce background signal. However, these tools are still imperfect and can generate false results. We have found that depending on the backbone of the reporter vector, pGL3 or pGL2, different results are obtained for a eukaryotic promoter's activation by metabolic changes. These observations were done in the course of investigation of the MMP2 (matrix metalloproteinase-2) promoter regulation in response to inhibition of glycolysis.