The "ciliopathies" are a newly defined group of disorders characterized by 
defects in the structure or function of the cellular primary cilium. Patients 
with these disorders display variably expressive fibrocystic renal disease, 
retinal blindness, polydactyly, obesity, and brain dysgenesis as well as 
neurocognitive impairments. Joubert syndrome is a ciliopathy defined by 
cerebellar vermis hypoplasia, oculomotor apraxia, intermittent hyperventilation, 
and mental retardation. Recent evidence suggests important roles for the primary 
cilium in mediating a host of extracellular signaling events such as morphogen, 
mitogen, homeostatic and polarity signals. Based upon the clinical features of 
ciliopathies and cilia mediated signaling pathways, the data support a role for 
the primary cilium in modulating neurogenesis, cell polarity, axonal guidance 
and possibly adult neuronal function.