Cystine is a naturally occurring amino acid formed by the oxidation of two molecules of the amino acid cysteine. It plays a crucial role in protein structure and function, as it helps stabilize proteins through the formation of disulfide bonds. Cystine is found in various foods, including meat, eggs, and dairy products, and is important for maintaining healthy skin, hair, and nails. In the human body, cystine can be converted back to cysteine, which is essential for the synthesis of glutathione, a powerful antioxidant. Cystine levels can be affected by certain medical conditions, such as cystinuria, a genetic disorder that leads to the formation of kidney stones due to excess cystine in the urine.