S S bond isomerases, also known as disulfide bond isomerases, are enzymes that play a crucial role in protein folding and structure. These enzymes facilitate the reshuffling of disulfide bonds between cysteine residues within a protein, thereby affecting its overall conformation. Synonyms for S S bond isomerases include protein disulfide isomerase (PDI), thioredoxin domain-containing proteins, and redox-active proteins. PDI is perhaps the most commonly used term to refer to these enzymes, reflecting their ability to catalyze disulfide bond formation, reduction, and isomerization. Understanding the function and mechanism of S S bond isomerases can provide valuable insights into protein folding pathways, protein quality control, and potential therapeutic interventions.