The NM_00138 c.338C>G is a missense variant in FBN1 predicted to cause a substitution of a tyrosine by cysteine at amino acid 113 (p. Ser113Cys) within an EGF-like domain of the protein. Cysteine residues are believed to be involved in the formation of disulfide bridges which are essential for the protein structure and this variant may impact disulfide bonding (PM1). This variant was found in a proband with thoracic aortic aneurysm and ectopia lentis, which is a highly specific phenotype for Marfan syndrome (Internal data, PP4). This variant has been reported twice in ClinVar: once as likely pathogenic and once as uncertain significance (Variation ID: 222600). It has been reported in the literature in individuals with a clinical diagnosis or clinical features of Marfan syndrome (PMID 34281902, Invitae Clinvar entry, Blueprint Genetics ClinVar entry, internal data, PS4_Mod). This variant is not present in gnomAD (PM2_sup; https://gnomad.broadinstitute.org/ v2.1.1). Computational prediction tools and conservation analysis are inconclusive with regards to a possible impact on this variant protein function and structure (REVEL: 0.384). The constraint z-score for missense variants affecting FBN1 is 5.06 (PP2). 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_Mod, PM1, PM2_Sup, PP2, PP4