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 selenium, an essential trace element found in foods like nuts, fish, and grains. These proteins are crucial for maintaining cellular health and protecting against oxidative stress. Some well-known selenoproteins include glutathione peroxidases and thioredoxin reductases, which help neutralize harmful free radicals. A deficiency in selenium can lead to reduced levels of selenoproteins, potentially resulting in health issues.