Current concepts in the embryology and genetics of cleft lip and cleft palate.

Many mechanisms underlying normal and abnormal craniofacial embryogenesis are well understood. The genetic factors that provoke abnormal development and result in orofacial clefts are not clear, but much progress has occurred in our understanding. Genes or chromosomal rearrangements on many chromosomes can lead to syndromes that include orofacial clefts. This diversity in the mechanisms that can lead to syndromic clefts highlights the fact that the processes leading to the development of the oral cavity and face are complex and sensitive to disturbances at multiple timepoints or within multiple genetic domains. As for nonsyndromic clefting, large-scale family studies are consistent with one or a few loci exerting major effects on phenotypic expression, although no single gene has been identified as a "necessary" locus for development of nonsyndromic clefts. Rather, the emerging consensus is that the genetic etiology of nonsyndromic clefting is complex, with several loci showing significant results in at least some studies. Some of these loci may be genes for susceptibility to environmental factors, some may be modifying loci, and some may be "necessary" loci. Mutations in genes that are now known to control early development are logical candidate genes for future studies of nonsyndromic orofacial clefting. Continued genetic analyses and developmental studies are crucial for eventual understanding of the complex etiology of these common congenital anomalies.