Dear Editor,Insulin plays a critical role in mammalian glucose homeostasis,and dysfunctional insulin secretion results in diabetes.Insulin maturation in pancreatic β cells can be generally divided into three stages:the first stage is the biosynthesis of proinsulin in the endoplasmic reticulum (ER),the second stage is the transport of proinsulin from the ER into the Golgi,and the third stage is the cleavage of proinsulin into insulin and C peptide in immature insulin granules (Dodson and Steiner,1998).