Paradoxically, patients with ADPKD also present with high circulating levels of arginine vasopressin (AVP; and its more stable surrogate, copeptin) (5–8), which binds to the vasopressin type 2 receptors (V2Rs) to increase water reabsorption in the collecting duct (CD) through the apical incorporation of aquaporin 2 (AQP2). The gene discussed is AVP; the disease is autosomal dominant polycystic kidney disease.