These drugs, which include angiostatin and Endostatin, halt the process of developing new blood vessels (angiogenesis). Angiostatin is a piece of a larger and very common protein, plasminogen, that the body uses in blood clotting. Endostatin is a piece of a different protein, collagen 18, that is in all blood vessels. Both angiostatin and Endostatin are normally secreted by tumors. It is hoped that they will provide the basis for a new class of agents to treat cancer.