Model for the role of MARK4 in primary cilium formation. Primary cilium formation is initiated by the fusion of vesicles to the mother centriole, which thus becomes a basal body. Subsequently, the ciliary axoneme elongates from the basal body. MARK4 is not involved in vesicle docking but is crucial for the initiation of axoneme extension. MARK4 most likely regulates ODF2 functions but may also act on other targets to promote ciliogenesis. A decrease in MARK4 activity blocks ciliogenesis after the initial vesicle docking and before the complete displacement of the inhibitory proteins CP110 and Cep97.