Irradiation of endothelial cells in vitro induces the surface exposure of anionic phospholipids, principally phosphatidylserine (shown in green in the figure). Irradiation of human lung tumors in mice likewise induces exposure of phosphatidylserine on tumor vascular endothelium. Targeting the exposed phosphatidylserine with a monoclonal antibody (2aG4) leads to host-cell mediated destruction of tumor vessels and reductions in tumor growth in the mice. Bavituximab, a chimeric version of 2aG4 in clinical trials, has the potential to improve the efficacy of radiation therapy in cancer patients by amplifying damage to tumor vasculature. For details, see the article by He and associates 5211 of this issue.