Dual use of the centrioles during cell cycle and primary cilium formation. In most cells, primary cilium formation first occurs during the G1 phase following centrosomal docking to the membrane. Intraflagellar transport (IFT) and accessory proteins build the ciliary axoneme, which extends directly from the mother centriole's triplet microtubules. During this stage of the cell cycle, as well as during the G0 phase, the cilium functions as a cellular antenna, interpreting extracellular signals such as Hedgehog and platelet-derived growth factor (PDGF). Upon entry into the S phase, the cell's centrioles and the DNA begin to replicate. The centrioles reach maturity during the late G2 phase, at which point the cilium is disassembled so that the engaged centrioles can be liberated for mitotic spindle formation. Once cell division is complete, the centrioles can proceed to ciliary reassembly in G1. Reprinted with permission .