NM_000138.5 c.6541T>C is a missense variant in FBN1 predicted to cause a substitution of cysteine by an arginine at amino acid 2181 (p.Cys2182Arg). It has been identified in one patient meeting clinical diagnostic criteria for Marfan syndrome, including thoracic aortic aneurysm and dissection (TAAD) and systemic involvement, and was found to segregate with TAAD and systemic features in the patient’s sibling (PP4; University of Tokyo). This variant has also been reported in two patients with TAAD, without additional phenotypes noted (PS4_supporting; PMID: 37042257, GeneDx internal data). It is absent from gnomAD (PM2_supporting; https://gnomad.broadinstitute.org/, v2.1.1, 3.1.2, 4.0.0). This variant affects a cysteine residue in a calcium-binding EGF-like domain; cysteine residues are involved in the formation of disulfide bridges which are essential for the protein structure (PM1_strong). Computational prediction tools and conservation analysis support that this variant is likely to impact the protein (PP3; REVEL = 0.993). The constraint z-score for missense variants affecting FBN1 is 8.18 (PP2; gnomAD v4.0.0). 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: PM1_strong, PP2, PP3, PP4, PS4_supporting, PM2_supporting.