During hyperglycemic clamps at 3 glucose levels (7.5, 10 and 15 mmol/l), the first and second phase insulin responses were impaired in those with diabetes, and treatment with a sulphonylurea (gliclazide) in those with diabetes resulted in restoration of both the first and second phase insulin responses, with an approximate doubling of the insulin and C-peptide response at each level of clamp. The gene discussed is INS; the disease is diabetes mellitus.