Studies in a mouse model of KATP hyperinsulinism, the most common and severe genetic subtype of hyperinsulinism, have shown that the GLP-1 receptor antagonist exendin-(9-39) significantly increases fasting plasma glucose and decreases the insulin to glucose ratio on the fasting state, effectively reversing the hyperinsulinemic hypoglycemia phenotype. The gene discussed is GLP1R; the disease is hyperinsulinism.