The discovery of the ciliary localization and functions of polycystin-1 and polycystin-2, the products of the ADPKD genes PKD1 and PKD2, led to the hypothesis that cilia control oriented cell division and proliferation of tubular cells, and that ciliary defects therefore result in cystic kidneys, which became a classic sign of ciliopathies. Here, PKD1 is linked to autosomal dominant polycystic kidney disease.