Autosomal dominant polycystic kidney disease (ADPKD) is a common genetic disease that affects both adults and children. Renal cysts are the cardinal sign of the disease that also causes cysts in liver, pancreas, testis, and ovary, as well as cardiac valvular insufficiency and arterial aneurysms. At least three genes cause ADPKD in humans. PKD1 and PKD2 have been cloned and sequenced, both code for novel proteins. Analyses of their primary structures suggest that polycystin-1, the PKD1 gene product, is a receptor, while similarities between the polycystins and calcium channel subunits suggest that these proteins are subunits of a novel channel. Individuals with mutations in PKD1 or PKD2 have identical phenotypes, which present at a later age in PKD2 patients. Recent evidence suggests that the two polycystins interact, providing a biochemical basis for the similarity of disease caused by mutations in PKD1 and PKD2. Consistent with its protean manifestations, polycystin-1 is widely expressed in both epithelial and non-epithelial tissues during embryological development. Mice with targeted mutations of either the PKD1 or the PKD2 genes die during embryogenesis. Thus, the PKD genes are required for normal fetal development. The observation that loss of polycystin-1 or -2 function causes death during embryogenesis suggests that PKD1 and PKD2 might be part of a morphoregulatory pathway.