PKD2 and autosomal dominant polycystic kidney disease: The best studied examples of ciliary membrane proteins include the autosomal dominant polycystic kidney disease (ADPKD) gene products polycystin-1 (PKD1; OMIM: 601313) and polycystin-2 (PKD2; OMIM: 173910), which form a cilium-localized heterodimeric receptor-cation channel complex essential for preventing cystogenesis 78–81, and the Sonic Hedgehog (SHH) co-receptor Patched-1 (PTCH1) (OMIM: 601309) and the class F GPCR, Smoothened (SMO) (OMIM: 601500), which accumulate in cilia in the absence and presence of SHH, respectively 82–84 (Figure 2b).