Hypoxic downregulation of Hif1a exon I.2 promoter activity. Exon I.2 upstream sequences of various length as indicated were placed in front of the firefly luciferase reporter gene. These constructs were transiently transfected into mouse L929 fibroblasts or Hepa1 hepatoma cells, followed by exposure to normoxia or hypoxia for 36 to 38 hours. Subsequently, luciferase expression was determined as described in Fig 7. All values were normalized to the respective normoxic luciferase activity obtained with the construct containing the longest 5′ region, which was arbitrarily defined as 100. Mean ± SD of three independent experiments are shown.