Tolvaptan, a vasopressin V2-receptor antagonist, suppresses binding of vasopressin to the V2-receptor in renal epithelial cells, and vasopressin V2-receptor blockade has been shown, in animal models of ADPKD, to inhibit the growth of renal cysts and the decline in renal function [5–9]. The gene discussed is AVPR2; the disease is autosomal dominant polycystic kidney disease.