Dermal fibroblasts from nine Marfan syndrome patients with missense mutations in the fibrillin–1 gene (FBN1) produced nearly normal amounts of fibrillin as determined by quantitative pulse–chase experiments. However, six of the seven mutations involving substitutions of highly conserved cysteine residues exhibited lower rates of intracellular transport and secretion. This effect is likely due to improper folding, since intracellular fibrillin processing was also affected by the reducing agent dithiothreitol. Normal secretion patterns were seen in three mutations that either change the conformation of EGF–like domains or change consensus amino acids required for Ca^{+ +}-binding. In all nine fibroblast strains, however, the deposition of fibrillin in the extracellular matrix was reduced to 50% of normal in two and to less than 30% in seven of the nine samples studied. The protein alterations caused by these missense mutations are associated with moderate to severe features of Marfan syndrome and a dominant negative mechanism is suggested to play a major role in their pathogenesis.