Calnexin is a new type of molecular chaperone that interacts with many nascent membrane and soluble proteins of the secretory pathway. Calnexin is unrelated to molecular chaperones of the Hsp60, Hsp70 and Hsp90 families, and is further distinguished from them in that it is an integral membrane protein. One of its demonstrated functions is the retention of incorrectly or incompletely folded proteins, suggesting that calnexin is a component of the quality control system of the endoplasmic reticulum.