So some proteins, specifically those proteins that are destined to be in the extracellular environment, these polypeptides, can be cross linked by these disulfide bonds. And once we form the unit of these two cystine molecules, that is known as acsteine. So this spelling and this spelling are not the same. So this is pronounced cystine. This is pronounced cysteine. So we have the hydrophobic interactions that basically drive the formation of that tertiary structure.