Essential oxidoreductase that oxidizes proteins in the endoplasmic reticulum to produce disulfide bonds. Acts by oxidizing directly PDI isomerase through a direct disulfide exchange. Does not act as a direct oxidant of folding substrate, but relies on PDI to transfer oxidizing equivalent. Does not oxidize all PDI related proteins, suggesting that it can discriminate between PDI and related proteins. Its reoxidation probably involves electron transfer to molecular oxygen via FAD. Acts independently of glutathione. May be responsible for a significant proportion of reactive oxygen species (ROS) in the cell, thereby being a source of oxidative stress (By similarity). May function both as a monomer and a homodimer. A number of isoforms are produced. According to EST sequences. N-glycosylated. Belongs to the EROs family.