Cancer is defined as any of a group of diseases in which particular cells in a body cease to respond to normal growth controls. The cells multiply unchecked, crowding out, invading, and destroying other tissues. One of the most important discoveries in cancer research in recent years are the genes that scientists think promote this unrestricted growth, called oncogenes. Experts believe that oncogenes alter receptor molecules located on the surface of cells that are responsible for signaling the cell to divide. These receptors somehow get stuck in the "on" position, sending signals to the cells to replicate at a rate that far exceeds cell loss.

Oncogenes and faulty receptors are certainly critical to the formation of cancerous tumors. Studies show, however, that the blood vessels that feed a growing tumor are just as important. Without a steady supply of oxygen and nutrients, cancerous tissues grow extremely slowly, are unable to spread throughout the body, or die out altogether.

The circulatory system must extend to all living tissues within the body. Therefore, wherever the body is undergoing development, growth, or repair, it must also grow a network of new blood vessels in a process called angiogenesis. Cancerous tissues are no different, although they promote angiogenesis somewhat indirectly. Tumor cells send chemical signals, called activator molecules, to the host's healthy cells. These chemical signals activate genes in the healthy tissue that, in turn, encourage the growth of new blood vessels into and around the cancerous tissue.

Medical researchers are now using this knowledge in their search for a cure for cancer. In one study, injections of a protein called endostatin, known to inhibit angiogenesis, greatly reduced the number of cancer cells and the size of tumors in a group of laboratory mice. Whether or not similar drugs can safely prevent the growth and spread of cancer cells in humans has been the focus of dozens of ongoing clinical trials since the late 1990s. While cancer researchers are cautious with their predictions, they say that results of these trials may lead to successful treatments of some forms of cancer.