Abstract
Recent studies of the primary cilium have begun to provide further insights into ciliary ultrastructure, with an emerging picture of complex compartmentalization and molecular components that combine in functional modules. Many proteins that are mutated in ciliopathies are localized to the transition zone, a compartment of the proximal region of the cilium. The loss of these components can disrupt ciliary functions such as the control of protein entry and exit from the cilium, the possible trafficking of essential ciliary components, and the regulation of signaling cascades and control of the cell cycle. The discovery of functional modules within the primary cilium may help in understanding the variable phenotypes and pleiotropy in ciliopathies.