The connecting peptide, or C-peptide, is a 31-aa long peptide that connects insulin A-chain to the B-chain in the proinsulin molecule. In the insulin synthesis pathway, preproinsulin is translocated into the endoplasmic reticulum of beta cells of the pancreas with an A-chain, a C-peptide, a B-chain, and a signal sequence. After signal sequence cleavage, the C-peptide is removed from the newly formed proinsulin in the Golgi apparatus leaving the A- and B-chains bound together by disulfide bonds, forming the insulin molecule. Besides a role in assembly, folding, and processing of insulin in the ER, the C-peptide is a bioactive peptide with effects on microvascular blood flow and tissue health. Because equimolar amounts of C-peptide are stored in secretory granules of the pancreatic beta cells and eventually released in the circulation, C-peptide is a marker of insulin secretion in understanding the pathophysiology of type 1 and type 2 diabetes. Rodents express 2 nonallelic genes of insulin. Insulin-2 is the rodent homologue of the human insulin. Insulin-1 lacks the second intron of the insulin-2 gene.Rat: H-Glu-Val-Glu-Asp-Pro-Gln-Val-Pro-Gln-Leu-Glu-Leu-Gly-Gly-Gly-Pro-Glu-Ala-Gly-Asp-Leu-Gln-Thr-Leu-Ala-Leu-Glu-Val-Ala-Arg-Gln-OH or H-EVEDPQVPQLELGGGPEAGDLQTLALEVARQ-OH