Comparison of the Hif1a exon I.1 and exon I.2 promoter activities. The empty parental vector pGL3Basic or the same vector containing either the exon I.1 promoter or the exon I.2 promoter were transiently transfected into mouse L929 fibroblast and Hepa1 hepatoma cells. Luciferase activity was determined after 24 to 28 hours of normoxic incubation. A cotransfected β-galactosidase expression vector served as internal control for transfection efficiency and extract preparation. All values were normalized to the respective luciferase activity obtained with the empty vector pGL3Basic, which was arbitrarily defined as 1. Mean ± standard deviation (SD) of three independent experiments are shown. Note the different scales.