During their synthesis, secretory proteins translocate into the endoplasmic reticulum (ER) 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. Protein disulfide isomerase (PDI) catalyzes the formation and isomerization of these disulfide bonds. Studies on mechanisms of oxidative folding suggest that molecular oxygen oxidizes the ER-protein Ero1, which in turn oxidizes PDI through disulfide exchange. This event is then followed by PDI-catalyzed disulfide bond formation in folding proteins.