Until recently, the nature of the molecules involved in inherited cystic disease of the kidney remained unknown. These diseases are characterized by the development of multiple abnormal fluid-filled sacs or dilations in the kidney parenchyma, often leading to significant renal failure. The recent characterization of the PKD1 gene product and of other genes involved in murine polycystic models underscores the complexity of the pathways that lead to renal cystic disease.