NM_00138 c.1825C>T is a missense variant in FBN1 predicted to cause a substitution of an arginine by cysteine at amino acid 609 (p.Arg609Cys). This variant was found in a proband with thoracic aortic aneurysm and dissection (TAAD) and a proband with minor skeletal features (PMID: 32154576; Bichat internal data). This variant is present in gnomAD (BS1; 2/18384 [0.011%] East Asian alleles; https://gnomad.broadinstitute.org/). It has been submitted to ClinVar twice, once as likely pathogenic and once as of uncertain significance (Variation ID: 439709). This variant introduces a novel cysteine residue which may impede the normal formation of critical disulfide bridges (PM1). Computational prediction tools and conservation analysis suggest that this variant may impact the protein’s structure or function (PP3). In summary, this variant meets criteria to be classified as likely pathogenic for Marfan syndrome based on the ACMG/AMP criteria applied, as specified by the ClinGen FBN1 VCEP: Due to the conflicting and insufficient evidence, this variant is classified as uncertain significance for Marfan syndrome based on the ACMG/AMP criteria applied, as specified by the ClinGen FBN1 VCEP: BS1, PM1, PP3.