Cystine is the amino acid with the formula (SCH₂CH(NH₂)CO₂H)₂. It is a white solid that is slightly soluble in water. Each molecule of cystine is made from two molecules of cysteine, another sulfur-containing amino acid that, as its name suggests, is very similar to cystine, with which it can be interconverted. It serves two biological functions, a site of redox reactions and a mechanical linkage that allows proteins to retain their 3-dimensional structure. It is a nonessential amino acid, which means that the human body can synthesize it.