NM_000138.5 c.6254G>A is a missense variant in FBN1 predicted to cause a substitution of a cysteine by tyrosine at amino acid 2085 (p.Cys2085Tyr). This variant was found in a proband with a phenotype that satisfies the revised Ghent criteria for a clinical diagnosis of Marfan syndrome (MFS) (PP4; PMID: 25907466). It has been identified in another individual who meets clinical diagnostic criteria for MFS and one who does not meet diagnostic criteria but has thoracic aortic aneurysm (TAA) and a systemic score of 4 (PS4_moderate; PMID: 29768367; Bichat internal data). It has been reported 2 times in ClinVar as likely pathogenic (1) and of uncertain significance (1) (Variation ID: 547338). It is not present in gnomAD (PM2_supporting; https://gnomad.broadinstitute.org/). This variant alters a cysteine in a TGFβ binding-protein-like (TB) domain, in which cysteine residues are believed to form disulfide bridges important for proper protein folding (PM1). Computational prediction tools and conservation analysis suggest that this variant may impact the protein structure (PP3; REVEL = 0.886). A different variant at the same amino acid position (c.6253T>G:p.Cys2085Gly) has been identified in at least two individuals with TAA and ectopia lentis and is classified as pathogenic or likely pathogenic; this further supports the functional importance of this amino acid position (PM5; Invitae internal data). The constraint z-score for missense variants affecting FBN1 is 8.2 (PP2; gnomAD v4.1.0). 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, PM5, PP2, PP3, PP4, PM2_supporting, PS4_supporting.