Hypoxia-inducible factor-1 (HIF-1) stabilization is a pivotal event in the response to hypoxic stress. A study in the December 15, 2006 issue of Cancer Research shows that HIF-1 stabilization occurs more robustly as a result of intermittent hypoxia compared with chronic hypoxia. The findings of this study suggest that intermittent hypoxia might influence the efficacy of radiotherapy by more strongly affecting the growth and survival of vascular endothelial cells. This finding offers additional encouragement to efforts to target HIF-1 for cancer therapy.