Before it was found to be an essential nutrient (3–4), selenium was considered highly toxic to animals and humans (5). The key breakthrough occurred in 1973, when Rotruck and colleagues (3) discovered that selenium protected against oxidative damage by means of selenium-dependent glutathione peroxidase. Selenium is incorporated into selenoproteins as selenocysteine through a complex genetic mechanism encoded by the UGA codon (6). Selenoproteins, including glutathione peroxidases, thioredoxin reductases, iodothyronine deiodinases, and selenoprotein P, have important enzymatic functions. Through selenoproteins, selenium is involved in many biological functions, including protection against oxidative stress, immune function, and thyroid function (6–7).