Selenium is an essential trace mineral and functions as part of the antioxidant enzyme glutathione perioxidase.  It is bonded to the essential sulfur amino acid, methionine, in a naturally occuring form as found in the diet.  Selenium is found most abundantly in Brazil Nuts, organ meats, seafood, and wheat germ.*

From the FDA's website: "Selenium may reduce the risk of certain cancers. Some scientific evidence suggests that consumption of selenium may reduce the risk of certain forms of cancer. However, FDA has determined that this evidence is limited and not conclusive."