Selenium-containing proteins, also known as selenoproteins, are proteins that incorporate the amino acid selenocysteine, which contains selenium. These proteins play crucial roles in various biological processes, including antioxidant defense, thyroid hormone metabolism, and immune function. Selenoproteins are essential for human health, as they help protect cells from oxidative damage and support metabolic functions. The most well-known selenoprotein is glutathione peroxidase, which helps reduce harmful peroxides in the body. Deficiencies in selenium or selenoproteins can lead to various health issues, including weakened immune response and increased risk of chronic diseases.