The NM_00138 c.3290G>A, is a missense variant in FBN1 predicted to cause a substitution of a cysteine by tyrosine at amino acid 1097 (p.Cys1097Tyr). This variant was found in a proband with a neonatal onset of Marfan syndrome (internal data, PP4). This variant has been reported four times in ClinVar: once as pathogenic, twice as likely pathogenic, and once as uncertain significance (Variation ID: 549150). This variant has been reported in the literature in individuals with clinical diagnosis of Marfan syndrome and/or features of Marfan syndrome (PMID 37684520, 29357934, Invitae ClinVar entry, internal data; PS4_Mod), and was observed de novo without parental confirmation in one individual with a highly specific phenotype (PMID 37684520; PM6). This variant is not present in gnomAD (PM2_sup; https://gnomad.broadinstitute.org/ v2.1.1). This variant affects a cysteine residue in a calcium binding EGF domain. Cysteine residues 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: 0.979, PP3). The constraint z-score for missense variants affecting FBN1 is 5.06 (PP2). 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, PM6, PM2_Sup, PP2, PP3, PP4.