The biosynthesis of insulin occurs in the β-cell via a single-chain protein, proinsulin, which is converted to insulin by cleavage of a peptide that connects the COOH terminus of the B chain to the NH2 terminus of the A chain. The primary structure of porcine proinsulin is known. Differences in the sequences of the porcine and bovine connecting peptides are reflected by immunologic differences between the respective proinsulins. Proinsulin can be detected in the urine and blood of certain diabetic individuals and may be related to another form of blood insulin, "big" insulin. Although much less effective, proinsulin mimics the actions of insulin on isolated adipose and diaphragm tissues. This is not an intrinsic activity but the result of tissue conversion to a molecule(s) with insulin-like effects.