Effect of a hypoxic enhancer on KDR promoter activity after retroviral delivery. Murine TNF-α and firefly luciferase were used as reporter genes, generating the viruses SKMT, HRE-SKMT and HRE-SKL. Levels of secreted mTNF-α protein or luciferase in stably transfected cell lines were measured after 16 h exposure to normoxia/hypoxia (0.1% O2). The mean of five independent experiments is shown ± s.d. For the calculation of luciferase the expression under normoxic conditions was set to one-fold. The 2.3-fold increase of expression in HRE-SKL infected cells under hypoxia is significant (P < 0.05).