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Most proteins are chemically modified after they are made to control how, when, and where they function. It was thought that only the amino acid side chains and the peptide bonds of proteins were chemically modified. It has become clear in recent years that the disulfide bonds of proteins are also modified. Some disulfide bonds are cleaved in the mature protein to control function. From a few examples a decade ago, there are now about thirty well-defined examples of control of protein function by functional disulfides and dozens more examples that are under investigation. Cleavage of a functional disulfide in a protein changes the bioactivity of the protein. Changes in ligand binding, substrate hydrolysis, proteolysis, or oligomer formation have been described. The functional disulfides are cleaved by reductases or by thiol–disulfide exchange. Viral, bacterial, plant and mammalian proteins are regulated by functional disulfides, so this mechanism of protein control operates in all life forms. The early indications are that cleavage of disulfide bonds may rival proteolysis of peptide bonds as a means of protein control.