Fig. 3. Functions of primary cilia. The primary cilium acts as a sensory organelle that transfers information from the extracellular environment to the cell interior. For example, cation channels composed of the polycystin proteins Pc1and Pc2 in the ciliary membrane sense mechanical stress, while receptors such as the platelet-derived growth factor receptor (Pdgfr) sense extracellular ligands. The processing and transfer of signaling information to the cell is mediated by specialized proteins, including smoothened (Smo), and several microtubule-associated protein complexes that include members of the nephronophthisis protein family [Nphp, inversin (Invs)], and proteins associated with Bardet-Biedl syndrome (BBS). Signals from primary cilia ultimately are involved in regulating crucial cellular processes, including the cell cycle, cytoskeletal organization, intraflagellar transport and signaling pathways, such as the hedgehog, canonical Wnt and non-canonical Wnt/planar cell polarity (PCP) pathways. Abbreviations: Apc2, anaphase promoting complex protein 2; Dvl1, dishevelled; Nek8, NIMA-related kinase 8; Ofd1, oral-facial-digital type 1 protein; Pcm1, pericentriolar material protein 1.