Cystine is the oxidized dimer form of the amino acid cysteine and has the formula (SCH₂CH(NH₂)CO₂H)₂. It is a white solid that is slightly soluble in water. It serves two biological functions: a site of redox reactions and a mechanical linkage that allows proteins to retain their three-dimensional structure. It's the main raw material of l-cysteine Hydrochloride.