The findings that blockade of GLP-1 signalling abolished metformin-induced suppression of hepatic glucose output in rodents [13], and markedly attenuated the effect of metformin in reducing the glycaemic response to a mixed meal in people with type 2 diabetes [14], attest to a major role for GLP-1 in mediating the glucose-lowering effect of metformin. The gene discussed is GCG; the disease is type 2 diabetes mellitus.