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Hypoxia is a common feature in tumors associated with an increased resistance of tumor cells to therapies. In addition to O₂ diffusion–limited hypoxia, another form of tumor hypoxia characterized by fluctuating changes in pO₂ within the disorganized tumor vascular network is described. Here, we postulated that this form of intermittent hypoxia promotes endothelial cell survival, thereby extending the concept of hypoxia-driven resistance to the tumor vasculature. We found that endothelial cell exposure to cycles of hypoxia reoxygenation not only rendered them resistant to proapoptotic stresses, including serum deprivation and radiotherapy, but also increased their capacity to migrate and organize in tubes. By contrast, prolonged hypoxia failed to exert protective effects and even seemed deleterious when combined with radiotherapy. The use of hypoxia-inducible factor-1α (HIF-1α)–targeting small interfering …