Our knowledge of selenium deficiency as a cause of animal disease has increased since 1973, when the role of selenium (Se) in glutathione peroxidase was discovered. Glutathione peroxidase (GSH-Px) works with vitamin E in the cytosol to prevent cell membrane damage. Selenium deficiency causes nutritional myopathy (white muscle disease) in young calves and yearlings. It is also implicated in a sudden death syndrome in calves less than three months old from acute myocardial degeneration.