The measurement of blood C-peptide and insulin has several clinical uses: The investigation of conditions resulting in excessive endogenous insulin secretion, as found in insulinoma [1] and in investigating the causes hypoglycaemic episodes (i.e. endogenous vs. exogenous source of insulin) [2]; the assessment of endogenous insulin secretion in people with diabetes to help define aetiology and best treatment [3], [4], [5]; and the assessment of insulin resistance [6]. The gene discussed is INS; the disease is pancreatic insulinoma.