One class of angiogenesis inhibitors being tested in cancer patients are molecules that directly inhibit the growth of endothelial cells. Included in this category is endostatin, the naturally occurring protein known to inhibit tumor growth in animals. Another drug, combretastatin A4, causes growing endothelial cells to commit suicide (apoptosis). Other drugs, which interact with a molecule called integrin, also can promote the destruction of proliferating endothelial cells.