PKD1 and cystic kidney disease: The protein products of PKD1 and PKD2 are transmembrane proteins called polycystin-1 (TRPP1, PC1) and polycystin-2 (TRPP2, PC2), respectively, whose localization to primary cilia was an essential clue in directly linking primary cilia to renal cysts (Yoder et al., 2002).