Many congenital malformations, such as cleft palate and neural tube defects, have a multifactorial origin involving both environmental and genetic factors. Conditions such as these may be exclusively monogenic, polygenic or environmental, but in most cases both genetic and environmental factors are involved. This study describes the sub-chromosomal localization of a single gene defect causing cleft palate and ankyloglossia (tongue-tied) in a large Icelandic family. This defect is a model for the analysis of other neural-crest malformations that show a more complex multifactorial inheritance pattern.