Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic cause of end-stage renal disease, affecting approximately 1 in 1,000 people. The disease is characterized by the development of numerous large fluid-filled renal cysts over the course of decades. These cysts compress the surrounding renal parenchyma and impair its function. Mutations in two genes are responsible for ADPKD. The protein products of both of these genes, polycystin-1 and polycystin-2, localize to the primary cilium and participate in a wide variety of signaling pathways. Polycystin-1 undergoes several proteolytic cleavages that produce fragments which manifest biological activities. Recent results suggest that the production of polycystin-1 cleavage fragments is necessary and sufficient to account for at least some, although certainly not all, of the physiological functions of the parent protein.