Folding of disulfide-bonded proteins in vitro and in vivo. In vitro: (A) under reducing conditions, which begin with a reduced, unfolded protein and include a reductant such as β-ME; (B) under non-reducing conditions, starting from an oxidized, unfolded protein; (C) under oxidizing conditions, starting from a reduced, unfolded protein. Possible outcomes are shown. In vivo: (D) formation of a disulfide bond between sequential cysteines; (E) formation of a disulfide bond between non-sequential cysteines, with a PDI retaining them in a folding-competent state; (F) formation of an erroneous disulfide bond between sequential cysteines – these can be isomerized by PDI to allow formation of the correct bond between non-sequential cysteines; (G) initial formation of non-native disulfide bonds that are needed to form native structure with the support of a PDI as indicated.