Lanthionization is the process by which hydroxide relaxers permanently straighten hair. It breaks the hair's disulfide bonds during processing and converts them to lanthionine bonds when the relaxer is rinsed from the hair.
Disulfide bonds contain two sulfur atoms. Lanthionine bonds contain only sulfur atom. The disulfide bonds that are broken by hydroxide relaxers are broken permanently and can never be re-formed.