This research study encompasses a range of phenotypes that include Pallister-Hall syndrome, the allelic disorder Greig cephalopolysyndactyly syndrome (GCPS), McKusick-Kaufman syndrome (MKS), and Bardet-Biedl syndrome (BBS). The clinical manifestations of these disorders include polydactyly, central nervous system malformations (with or without mental retardation and seizures), craniofacial malformations, and visceral malformations such as renal malformations or congenital heart defects. We study these disorders by a translational approach that begins in the clinic with careful clinical evaluation of the phenotypes by physical examination, imaging studies that include radiographs, ultrasound, MRI and CT scanning. We have shown that BBS and MKS can both be caused by mutations in the same gene. PHS and GCPS are caused by a wide spectrum of mutations in the GLI3 gene. One type of mutations causes PHS (3? truncations) and any loss of function mutation causes GCPS. The severity of the GCPS phenotype, specifically the mental retardation and learning disability, are correlated with the mutations. Patients with larger deletions have a more severe phenotype.