Proinsulin is a prohormone precursor to Insulin, made by the pancreas in response to increased blood sμgar. Proinsulin is 39 amino acids longer than the mature Insulin, of which 4 are completely lost and the remaining 35 forms the C-peptide. The measurement of Proinsulin in serum provides a useful, valuable information for the diagnosis of insulinomas. Proinsulin levels have also been shown to be elevated in non-insulin dependent diabetics (NIDDM), in insulin dependent diabetics (IDDM) and other clinical situations. The antibody recognizes insulin and recombinant proinsulin, but does not react with synthetic C-peptide.