Researchers have identified a compound that may cut off tumors' oxygen supply. Because they grow so rapidly, tumors eventually outgrow the ability of the surrounding blood vessels to transport enough oxygen and nutrients to them. In response to low oxygen levels, tumors trigger the formation of new blood vessels to keep them supplied. Now, scientists have discovered a protein, HIF-1, that acts as a 'master switch' that turns on hundreds of other genes involved in forming these new blood vessels. They then identified a new compound called cyclo-CLLFVY that blocked HIF-1 in cultured cancer cells. Researchers now hope to develop cyclo-CLLFVY into a drug that can prevent tumors from getting the oxygen they need to survive.