Selenium is a trace mineral which functions as a cofactor for an important antioxidant exzyme system in the body called glutathione peroxidase. It has been shown in one large study to reduce the risk of secondary cancers in a group of individuals with a previous diagnosis of skin cancer. The dose used in the study was 200 mcg/day. It is important to use an organic form of the mineral like selenomethionine rather other forms. Brazil nuts are very rich in selenium; one provides about 200 mcg.