Binding immunoglobulin protein (BIP) also known as 78kD glucose-regulated protein(GRP-78) or heat shock 70kD protein 5(HSPA5) is a HSP70 molecular chaperone located in the lumen of the endoplasmic reticulum(ER) that binds newly synthesized proteins as they are translocated into the ER, and maintains them in a state competent for subsequent folding and oligomerization. BIP is also an essential component of the translocation machinery, as well as playing a role in retrograde transport across the ER membrane of aberrant proteins destined for degradation by the proteasome. BIP is an abundant protein under all growth conditions, but its synthesis is markedly induced under conditions that lead to the accumulation of unfolded polypeptides in the ER.