Selenocysteine plays a crucial role in the function of selenoproteins, which are essential for various biological processes. It is incorporated into these proteins during translation, where it acts as the 21st amino acid. Selenoproteins are involved in antioxidant defense, helping to neutralize reactive oxygen species and protect cells from oxidative damage. This is particularly important in tissues that are highly susceptible to oxidative stress, such as the thyroid gland and the liver.

In the thyroid gland, selenocysteine-containing enzymes are vital for the metabolism of thyroid hormones, which regulate growth, development, and metabolism. Additionally, selenoproteins contribute to the immune system's function by modulating inflammation and supporting immune cell activity. They also play a role in DNA synthesis and repair, ensuring the integrity of genetic material.

In the field of biotechnology, selenocysteine is used to study protein structure and function due to its unique properties. It can be incorporated into proteins to enhance their stability or to serve as a probe in X-ray crystallography, aiding in the determination of protein structures. Overall, selenocysteine is indispensable for maintaining cellular health and has significant applications in both biological research and medical science.