Cysteine residues are sometimes referred to as half-cystines so that the formation of disulfide bridges may be considered to be brought about by the combination of two half-cystine residues. Disulfide bridges reduce the flexibility of a peptide, making it more rigid and decreasing the number of conformations available to it in solution. Such conformational constraint is often necessary for biological activity, which may be lost if the disulfide is disrupted. Disulfide bridges may be either intramolecular or intermolecular depending upon whether the cysteine residues involved are on the same chain or on different ones, respectively. Many naturally occurring peptides and proteins have one or more intramolecular disulfide bridges. The ease with which intramolecular disulfide bridges can be formed depends predominantly upon conformational considerations. If the peptide chain has only two cysteine residues, then only one intramolecular disulfide bridge is possible, but for peptides with more than two the situation becomes appreciably more involved.