Current preclinical and clinical knowledge of metformin action suggest that patients exhibiting hyperinsulinemia and tumors expressing the insulin receptor, LKB1, and TSC2 would benefit most from metformin therapy, while patients with normal circulating insulin levels and tumors lacking expression of the insulin receptor, LKB1, and TSC2 would likely be unresponsive to the drug. The gene discussed is INS; the disease is hyperinsulinism.