Primary cilia are not calcium-responsive mechanosensors. (a) The sensory epithelium of the inner ear contains hair cells that are required for hearing. The hair cells have chevron-shaped rows of actin-based microvilli, called stereocilia (by definition, not true cilia). At the tip of each chevron is a single cilium, called a kinocilium. Because a known mechanosensor (stereocilium) is adjacent to a true cilium, the Ca²⁺-responsive mechanosensitivity of the true cilium can be objectively tested. When flow bends both structures, [Ca²⁺] remains constant in the adjacent kinocilium. (b) In contrast, the established mechanosensitive stereocilia display robust Ca²⁺ increases in response to bending. Modified from Delling et al. (2016) with permission from Nature Publishing Group.