Defects in the PKD1 and PKD2 genes cause autosomal dominant polycystic kidney disease (PKD) in~ 1 in 1000 adults worldwide. These genes encode polycystin-1 and polycystin-2, which are membrane proteins thought to be involved in a calcium signal transduction cascade that controls epithelial proliferation and differentiation. Individuals with mutations in these genes develop cysts in the ducts and tubules of their kidneys. These cysts lead to kidney failure in~ 50% of affected individuals [1]. Similar to autosomal dominant PKD, autosomal recessive PKD also results in the formation of kidney cysts, but most human cases are caused by defects in a gene of unknown function [2]. How dominant and recessive PKD are related is unclear, but three recent findings suggest that both forms of the disease involve the non-motile primary cilia that extend from the kidney epithelial cells into the lumen of the collecting ducts and tubules. First, the Caenorhabditis elegans homologues of polycystin-1 and polycystin-2 are localized to the nematode’s sensory cilia [3]. Second, the defective gene in the