Selenoproteins are a group of proteins that contain the amino acid selenocysteine, which is sometimes referred to as the 21st amino acid. They play crucial roles in various biological processes, including antioxidant defense, thyroid hormone metabolism, and immune function. Selenoproteins are essential for maintaining overall health and are found in many organisms, including humans. The synthesis of selenoproteins requires selenium, an essential trace element that must be obtained through diet. Foods rich in selenium include Brazil nuts, fish, and whole grains. Deficiencies in selenium can lead to health issues, highlighting the importance of these proteins in human nutrition and physiology.