The middle segment of Proinsulin that is between the N-terminal B-chain and the C-terminal A-chain. It is a pancreatic peptide of about 31 residues, depending on the species. Upon proteolytic cleavage of Proinsulin, equimolar Insulin and C-peptide are released. C-peptide Immunoassay has been used to assess Pancreatic beta Cell function in diabetic Patients with circulating Insulin Antibodies or exogenous Insulin. Half-Life of C-peptide is 30 min, almost 8 times that of Insulin.