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, similar to other amino acids. 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 presence of specific sequences in mRNA, it can instead direct the incorporation of selenocysteine. This unique mechanism highlights the importance of selenium in human health and its involvement in several key enzymes.