Endoplasmic reticulum resident protein 27, also known as ER protein 27, C12orf46 and ERP27, is an endoplasmic reticulum luminal protein which is a member of the protein disulfide isomerase family. ERP27 contains one thioredoxin domain and does not contain a CXXC active site motif. ERP27 is widely expressed in many tissues; it has highest expression in pancreas, with lower levels in spleen, lung, kidney, thymus, and bone marrow. ERP27 interacts with PDIA3 and binds somatostatin-14 via hydrophobic interactions. ERP27 may act as a protease, protein disulfide isomerase, thiol-disulfide oxidase or phospholipase.

ERP27 contains 1 thioredoxin domain and is a noncatalytic member of the protein disulfide isomerase family. Protein disulfide isomerases (PDIs) constitute a family of structurally related enzymes which catalyze disulfide bonds formation, reduction, or isomerization of newly synthesized proteins in the lumen of the endoplasmic reticulum (ER). They act also as chaperones, and are, therefore, part of a quality-control system for the correct folding of the proteins in the same subcellular compartment. PDI has been found to have moderate effects (25-fold) on the rate of oxidative folding of proteins in vitro. Recombinant Human Protein Disulfide Isomerase is involved in disulphide-bond formation and isomerization, as well as the reduction of disulphide bonds in proteins. Recombinant PDI has been found to have moderate effects (25-fold) on the rate of oxidative folding of proteins in vitro. ERP27 is a widely expressed protein which localizes to the ER and may act as a protease, protein disulfide isomerase, thiol-disulfide oxidase or phospholipase. ERP27 doesn't contain a CXXC active site motif indicating that it is a catalytically redox-inactive member of the protein disulfide isomerase family.