Cystine is the oxidized dimer of the amino acid cysteine, consisting of two cysteine residues linked by a disulfide bond. It is a non-essential amino acid that plays a critical role in the structure and stability of proteins by forming disulfide bridges that maintain tertiary and quaternary protein structures. Abundant in keratin-rich tissues such as hair, skin, and nails, cystine contributes to their mechanical strength and resilience. It is biosynthesized from cysteine via oxidation and is involved in redox homeostasis and antioxidant defense mechanisms in cells. In nutrition, cystine serves as a source of cysteine and is used in dietary supplements for supporting skin health, detoxification, and immune function. Industrially, it finds applications in pharmaceuticals, cosmetics, and as a dough conditioner in baking. Due to its low solubility, it can form crystals in urine under certain conditions, potentially leading to cystine kidney stones in susceptible individuals. Cystine is stable under physiological conditions but can be reduced back to cysteine by reducing agents like glutathione.