<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-definition>The amino acid methionine with selenium substituting for the sulphur moiety. Methionine is an essential amino acid in humans, whereas selenium is a free-radical scavenging anti-oxidant, essential for the protection of various tissues from the damages of lipid peroxidation. As a trace mineral that is toxic in high doses, selenium is a cofactor for glutathione peroxidase, an anti-oxidant enzyme that neutralizes hydrogen peroxide. L-Selenomethionine is considered a safe, efficacious form of selenium and is readily bioavailable. Selenium may be chemoprotective for certain cancers, particularly prostate cancer. (NCI04)</n0:def-definition><n0:def-source>NCI</n0:def-source></n0:ComplexDefinition>