The NM_00138 c.8378A>G is a missense variant in FBN1 predicted to cause a substitution of a tyrosine by cysteine at amino acid 2793 (p.Tyr2793Cys), located within the C-terminal region of the protein. This variant was found in a proband with thoracic aortic dissection and a systemic score >7, which is a highly specific phenotype for Marfan syndrome (MFS) (Internal lab data, PP4). This variant has been reported three times in ClinVar: once as likely pathogenic, 2 times as uncertain significance (Variation ID: 42443). This variant has also been identified in at least 1 individual with a clinical diagnosis of MFS, as well as in individuals with clinical features of MFS (PMID 17663468, 34916231, 37558401, Invitae ClinVar, PS4_Moderate). A different missense variant at this position, c.8377T>C p.Tyr2793His, has previously been reported in individuals with MFS and/or MFS-related features (PMID 21542060, ClinVar), however the p.Tyr2793His variant has not yet been reviewed the FBN1 Variant Curation Expert Panel. This variant is not present in gnomAD (PM2_sup; https://gnomad.broadinstitute.org/ v2.1.1). Computational prediction tools and conservation analysis suggest that this variant may impact the protein (REVEL: 0.967, PP3). The constraint z-score for missense variants affecting FBN1 is 8.18 (PP2; https://gnomad.broadinstitute.org/ 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: PS4_Moderate, PM2_Sup, PP2, PP3, PP4.