Insulin is a peptide hormone that is critical for glucose homeostasis. The mature Insulin peptide is derived from Proinsulin, which includes the Insulin A and B chains connected by a peptide fragment (Insulin C-peptide). Proinsulin is processed within the endoplasmic reticulum of pancreatic Î² cells into equimolar ratios of mature Insulin and Insulin C-peptide. Proinsulin binds the Insulin Receptor A isoform (IR-A) with high affinity resulting in cell proliferation and migration. It has low binding affinity for the Insulin Receptor B isoform (IR-B) and the Insulin-like Growth Factor I Receptor (IGF R1).