Congenital contractural arachnodactyly (CCA) is an autosomal dominant disorder that is phenotypically similar to but genetically distinct from Marfan syndrome. Genetic-linkage analysis has implicated the fibrillin-2 gene (FBN2) as the CCA locus. Mutation analysis of two isolated CCA patients revealed missense mutations, indicating that defects in FBN2 may be responsible for this disorder. However, cosegregation of a mutant allele with the disease phenotype has not yet been established. We have investigated the primary cause of CCA in a large well-characterized kindred with five generations comprising 18 affected individuals. Previous studies demonstrated linkage of this family's CCA phenotype to FBN2. Mutation analysis of cDNA derived from the proband and her affected brother, using a nonisotopic RNase cleavage assay, revealed the partial skipping of exon 31. Approximately 25% mutant transcript is produced, which is apparently sufficient to cause a CCA phenotype. Sequence analysis of genomic DNA revealed an unusual base composition for intron 30 and identified the mutation, a g-26t transversion, in the vicinity of the splicing branch-point site in intron 30. Genomic DNA from 30 additional family members, both affected and unaffected, then was analyzed for the mutation. The results clearly demonstrate cosegregation of the branch-point mutation with the CCA phenotype. This is the first report of a CCA mutation in a multiplex family, unequivocally establishing that mutation in FBN2 are responsible for the CCA phenotype. In addition, branch-point mutations only very rarely have been associated with human disease, suggesting that the unusual composition of this intron influences splicing stability.