'Ciliopathies' are an emerging class of genetic multisystemic human disorders 
that are caused by a multitude of largely unrelated genes that affect ciliary 
structure/function. They are unified by shared clinical features, such as mental 
retardation, cystic kidney, retinal defects and polydactyly, and by the common 
localization of the protein products of these genes at or near the primary 
cilium of cells. With the realization that many previously disparate conditions 
are a part of this spectrum of disorders, there has been tremendous interest in 
the function of cilia in developmental signaling and homeostasis. Ciliopathies 
are mostly inherited as simple recessive traits, but phenotypic expressivity is 
under the control of numerous genetic modifiers, putting these conditions at the 
interface of simple and complex genetics. In this review, we discuss the 
ever-expanding ciliopathy field, which has three interrelated goals: developing 
a comprehensive understanding of genes mutated in the ciliopathies and required 
for ciliogenesis; understanding how the encoded proteins work together in 
complexes and networks to modulate activity and structure-function 
relationships; and uncovering signaling pathways and modifier relationships.