The connecting Peptide, or C-Peptide, is a short 31-amino-acid protein that connects insulin’s A-chain to its B-chain in the proinsulin molecule. Patients with diabetes may have their C-Peptide levels measured as a means of distinguishing Type 1 diabetes from Type 2 diabetes or Maturity Onset Diabetes of the Young (MODY). Serum C-Peptide levels correlate with endogenous insulin production and surviving β-cells and are present in equimolar amounts. Ultrasensitive assays reveal C-Peptide production persists for decades after Type 1 disease onset and remains functionally responsive in patients with advanced disease, whose β-cells function was thought to have ceased. C-Peptide levels are measured instead of insulin levels because C-Peptide can assess a person’s own insulin secretion even if they receive insulin injections, and because the liver metabolizes a larger and variable amount of insulin secreted into the portal vein but does not metabolize C-Peptide, which means that blood C-Peptide may be a better measure of portal insulin secretion than insulin itself.