The cilia contact cycle. We propose a cycle of cilia contact formation, maintenance, and termination. Movement of primary cilia enables the tip of one cilium to contact the shaft of a nearby cilium. If adhesion molecules are engaged, then a point contact is formed. A zipper-like extension of the region of overlap creates a long-lasting, overlapping, cilia-cilia adhesion. By similarity to Chlamydomonas flagellar adhesion, we speculate that the maintenance of contact is an active process. When the contact is terminated, cilia are again free to move and may encounter another cilium.