Protein disulfide isomerase, also known as proline 4-hydroxylase β The subunit (P4HB), cellular thyroid hormone-binding protein (p55), and glutathione-insulin transhydrogenase are peripheral membrane proteins belonging to the protein disulfide isomerase family. It contains two thioredoxin domains that catalyze the formation, breaking and rearrangement of disulfide bonds. P4HB was located near the CD4 rich area on the surface of lymphocytes. In the melanomsome component, P4HB reduces and activates the fusion properties of the HIV-1 gp120 surface protein, thereby allowing HIV-1 to enter the cell. P4HB acts as a chaperone, inhibiting the aggregation of misfolded proteins. P4HB may be involved with other molecular chaperones in the structural modification of TG precursors in hormone biosynthesis.