Primary cilium is a membrane-protruding immotile sensory organelle. It had been 
supposed that the cilium was a static organelle for long periods. However, 
recent studies have uncovered that the cilium is dynamically organized organelle 
in a cell cycle-dependent manner; it is formed during G0/G1 phase and resorbed 
when the cells enter cell division cycle. Despite the primary cilium is very 
short and its surface area is extremely small, the cilium possesses a few kinds 
of G protein-coupled receptors, growth factor receptors and ion channels. 
Therefore, it can function as a signaling receptor for selective bioactive 
ligands and mechanical stresses. Dysregulation of the ciliary dynamics is linked 
with hereditary disorders, so called "ciliopathy", with clinical manifestations 
of microcephaly, polycystic kidney, situs inversus, polydactyly, and so on. No 
effective medical treatment for the ciliopathies has been available. Increasing 
evidences about the molecular mechanisms of ciliary dynamics and ciliary 
functions have revealed that enormous number of molecules regulate a cycle of 
ciliogenesis, cilium-derived signaling, ciliary resorption and elimination. 
However, it is a fact that research progress is far inferior to the full 
disclosure of the molecular mechanisms. Further studies are required to clarify 
the pathogenesis of the cilipathies. Moreover, efficient medical treatments are 
expected to be developed by pharmacological approaches.