Dermal fibroblasts from a 13-yr-old boy with isolated skeletal features of the Marfan syndrome were used to study fibrillin synthesis and processing. Only one half of the secreted profibrillin was proteolytically processed to fibrillin outside the cell and deposited into the extracellular matrix. Electron microscopic examination of rotary shadowed microfibrils made by the proband's fibroblasts were indistinguishable from control cells. Sequencing of the FBN1 gene revealed a heterozygous C to T transition at nucleotide 8176 resulting in the substitution of a tryptophan for an arginine (R2726W), at a site immediately adjacent to a consensus sequence recognized by a cellular protease. Six other individuals in the proband's family had the FBN1 mutation that segregated with tall stature. None of the affected individuals have cardiac or ocular manifestations of the Marfan syndrome. This mutation identifies a putative site for profibrillin to fibrillin processing, and is associated with isolated skeletal features of the Marfan syndrome, indicating that the FBN1 gene is one of the genes that determines height in the general population. The cellular effect of the mutation may be equivalent to a "null" FBN1 allele and may define the phenotype associated with FBN1 "null" alleles.