Selenoproteins are a unique group of proteins that contain the amino acid selenocysteine, which is sometimes referred to as the 21st amino acid. They play essential roles in various biological processes, including antioxidant defense, thyroid hormone metabolism, and immune function. Selenoproteins are synthesized in the body using the selenium mineral, which is crucial for their formation and activity. These proteins are found in many organisms, including humans, and are vital for maintaining overall health. Some well-known selenoproteins include glutathione peroxidases and thioredoxin reductases, which help protect cells from oxidative damage. A deficiency in selenium can lead to reduced levels of selenoproteins, potentially resulting in health issues.