Selenocysteine is a naturally occurring amino acid that contains selenium, an essential trace element. It is often referred to as the 21st amino acid and is incorporated into proteins during translation. Selenocysteine plays a crucial role in various biological processes, particularly in antioxidant defense and thyroid hormone metabolism. This amino acid is encoded by the UGA codon, which typically signals the end of protein synthesis. However, in the case of selenocysteine, specific sequences in the mRNA allow for its incorporation into proteins. It is found in several important enzymes, including glutathione peroxidase and thioredoxin reductase, which help protect cells from oxidative damage.