A protein extracted from the venom of the Southern Copperhead, a poisonous snake. It does not kill cancer cells, but stops their growth, preventing metastases (the spread of cancer). Like the new anti-angeogenesis drugs in the news lately, Contortrostatin stops the growth of new blood vessels, something that is needed for the spread of cancer. This is a new drug and it may be awhile before it is on the market.