When tumors switch to the angiogenic phenotype and recruit new blood vessels, endothelial cells in these vessels express proteins on their luminal surface that are not produced by normal quiescent vascular endothelium. One of these proteins, alpha_. 133 integrin, when prevented from binding to its ligand, causes apoptosis of endothelial cells in proliferating microvessels'. In this issue, Pasqualini et al'report an in vivo phage display method that permits potent enough to block angiogenesis completely, large bulky tumors in mice can regress to microscopic dormant tumors4 because tumor growth is angiogenesisdependent.