A naturally occurring organoselenium compound found in many plants, including garlic, onions, and broccoli, with potential antioxidant and chemopreventive activities. Se-Methyl-seleno-L-cysteine (MSC) is an amino acid analogue of cysteine in which a methylselenium moiety replaces the sulphur atom of cysteine. This agent acts as an antioxidant when incorporated into glutathione peroxidase and has been shown to exhibit potent chemopreventive activity in animal models. or A substance that contains the element selenium (a nutrient that protects cells against damage) and is found in certain plants such as garlic and broccoli. Se-methyl-seleno-L-cysteine can act as an antioxidant and may help prevent or slow the growth of cancer cells. It is a type of amino acid.