Secretory proteins translocate into the endoplasmic reticulum (ER) after their synthesis where they are post-translationally modified and properly folded. To reach their native conformation, many secretory proteins require the formation of intra- or inter-molecular disulfide bonds. This process is called oxidative protein folding. Disulfide isomerase (PDI) has two thioredoxin homology domains and catalyzes the formation and isomerization of these disulfide bonds. Other ER resident proteins that possess the thioredoxin homology domains, including endoplasmic reticulum stress protein 57 (ERp57), constitute the PDI family. ERp57 interacts with calnexin and calreticulin and is suggested to play a role in the isomerization of disulfide bonds on certain glycoproteins.