Selenocysteine-containing enzymes that have been identified in mammals include the glutathione peroxidase family (GPX1, GPX2, GPX3, and GPX4), one or more iodothyronine deiodinases and two thioredixin reductases. Selenoprotein P, a glycoprotein that contains 10 selenocysteine residues per 43 kDa polypeptide and selenoprotein W, a 10 kDa muscle protein, are unidentified as to function. Levels of all of these selenocysteine-containing proteins in various tissues are affected to different extents by selenium availability. Increased amounts of selenoproteins observed in response to selenium supplementation were shown in several studies to correlate with increases in the corresponding mRNA levels. In general, selenoprotein levels in brain are less sensitive to dietary selenium fluctuation than the corresponding selenoprotein levels in other tissues.