Marfan syndrome is a connective-tissue disorder affecting cardiovascular, skeletal, and ocular systems. The major Marfan locus has been identified as the FBN1 gene on chromosome 15; this codes for the extracellular-matrix protein fibrillin, a 350-kD constituent of the 8-10-nm elastin-associated microfibrils. We identified five MFS patients who were heterozygous for an RsaI restriction-site dimorphism in the 3'UTR of the FBN1 gene. This expressed variation was used to distinguish the mRNA output from each of the two FBN1 alleles in fibroblast cultures from these five patients. Three of the patients were shown to produce< 5% of the normal level of FBN1 transcripts from one of their alleles. This null-allele phenotype was not observed in 10 nonmarfanoid fibroblast cell lines.