Differential diagnosis of pituitary vs. ectopic ACTH-dependent CS is based on the measurement of serum cortisol after suppression with 8 mg of dexamethasone (specificity 67%, sensitivity 79%), complemented with the corticotropin-releasing hormone stimulation (CRH) test (specificity 88–93%, sensitivity 91–100%). The gene discussed is CRH; the disease is Cowden syndrome 1.