The NM_00138 c.7775G>T, is a missense variant in FBN1 predicted to cause a substitution of a cysteine by phenylalanine at amino acid 2592 (p.Cys2592Phe). This variant has been reported three times in ClinVar: once as pathogenic, once as likely pathogenic, and once as uncertain significance (Variation ID: 423498). This variant is not present in gnomAD (PM2_sup; https://gnomad.broadinstitute.org/ v2.1.1). This variant has been identified in an individual with ectopia lentis and a systemic score >7 and was reported to be de novo without confirmation of parental relationships (internal lab data, PM6_Sup). At least two additional individuals with clinical features of Marfan syndrome carry this variant (internal lab data, Invitae ClinVar, PS4_Mod). In one individual with thoracic aortic aneurysm and skeletal features, the variant was found to segregate in an affected sister (Internal lab data). This variant affects a cysteine residue in a calcium binding EGF-like domain. Cysteine residues in these domains are believed to be involved in the formation of disulfide bridges which are essential for the protein structure (PM1_strong). Computational prediction tools and conservation analysis suggest that this variant may impact the protein (REVEL: 0.839, 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 pathogenic for Marfan syndrome based on the ACMG/AMP criteria applied, as specified by the ClinGen FBN1 VCEP: PM1_Strong, PS4_Moderate, PM2_Sup, PM6_Sup, PP2, PP3.