Pde1a mutants had a mild renal cystic disease and a urine concentrating defect (associated with upregulation of PDE4 activity and decreased protein kinase A dependent phosphorylation of aquaporin-2) on a wild-type genetic background and aggravated renal cystic disease on a Pkd2WS25/- background. This evidence concerns the gene PDE1A and cystic kidney disease.