Cystine is a naturally occurring amino acid formed when two molecules of the amino acid cysteine bond together. It plays a crucial role in protein structure and function, contributing to the stability of proteins through the formation of disulfide bonds. Cystine is found in various foods, including meat, eggs, and dairy products, and is important for overall health. In the human body, cystine is involved in several biological processes, including the synthesis of glutathione, an antioxidant that helps protect cells from damage. Deficiencies in cystine can lead to health issues, such as cystinuria, a genetic disorder that affects kidney function.