A schematic of the base of primary cilium showing the MC (basal body) and the proposed function of CC2D2A at SDA. The MC includes triplet MTs (purple cylinders) that continue as the doublet MTs of the axoneme (pink cylinders). The plus and minus ends of MTs are marked. The proteins localized to the cilium and MC are indicated. Nine extensions of each MT bundle at the border of MC and transition zone represent the SDA (blue–green–yellow pyramids) and distal appendages (blue rods). The proteins localized to SDA are shown in an order from centre to periphery, based on the current experimental evidence. The SDA anchors MT arrays and helps in the docking of transport vesicles at the base of cilium. The distal appendages help in tethering the MC to the plasma membrane, separating the cilium compartment from the cytoplasm. The green circle at the far end of the distal appendage, on the right side, represents a Rab8a-tagged vesicle. Rab8a may help tether plasma membrane to distal appendages with support from Odf2 and CC2D2A. Inset shows the top view of MC at the level of SDA, which are shown anchored to MT. The electron micrographs on the right show a cross-sectional view at the level indicated: doublet axoneme (top), at the level of appendages (middle) and triplet MC (basal body) (bottom).