A. As a measure for fibrillin synthesis, the two left lanes show labeled fibrillin (arrow head) detected in the cytosol of fibroblasts from a normal control (CR) and from Case 1 (774) after a 30 min pulse with 35S-cysteine. The amount in lane 774 is 55% of that in lane CR. In the right four lanes, the incorporation of pulse-labeled fibrillin (arrow head) into the extracellular matrix, after 8 hour and 20 hour chase periods, is severely reduced in Case 1 (774/8 and 774/20) compared to the normal control fibroblasts (C8 and C20). B. Allele-specific RNA analysis. All individuals are heterozygous for the C6236T RsaI polymorphism in the FBN1 3'UTR. RT-PCR products from fibroblast RNA samples were digested with RsaI and electrophoresed on an SSCA gel under denaturing conditions. The band labeled T represent the uncut T allele, the two bands labeled C represent the cut C allele, and the band marked by asterisk is constant. Samples 1 and 5 are normal controls, sample 2 has an exon 51 skipping mutation , and the sample in lane 4 is from Case 1 (774) with deletion of exons 42 and 43. Both these samples, as well as the one in lane 7 show the normal pattern of fragments indicating equal transcripts of both alleles. In contrast, sample 3 reveals predominant transcripts with the C allele, while in samples 6 and 8 only the T allele is detectable. Lane 9 shows slight skewing towards the C allele. Samples 3 and 8 have FBN1 mutations leading to premature stop codons. For samples 6,7 and 9, no FBN1 mutations have been identified as yet.