Patients with type 1 require accurate insulin dose replacement;10,11 patients with type 2 continue to produce substantial amounts of their own insulin, responding to non-insulin therapy, or if insulin is needed good control can be achieved with non-physiological insulin regimens.12,13 Measuring endogenous insulin secretion (using C-peptide, a component of the insulin pro-hormone secreted in equimolar amounts to insulin) in longstanding diabetes may be a useful ‘gold standard’ marker of endogenous insulin production, confirming a diagnosis of type 1 versus type 2 diabetes. The gene discussed is INS; the disease is diabetes mellitus.