Protein Disulfide-Isomerase (P4HB) is an endoplasmic reticulum lumen protein that belongs to the protein disulfide isomerase family. P4HB contains two thioredoxin domains and catalyzes the formation, breakage, and rearrangement of -S-S- bonds in proteins. P4HB is involved in hydroxylation of prolyl residues in preprocollagen. P4HB has the ability to act as a chaperone that inhibits aggregation of misfolded proteins in a concentration-dependent manner. P4HB plays a role in both the influx and efflux of S-nitrosothiol-bound nitric oxide.