NM_00138 c.6866G>C is a missense variant in FBN1 predicted to cause a substitution of cysteine by serine at amino acid 2289 (p.Cys2289Ser). This variant is not reported in the literature in any individuals with Marfan syndrome (MFS). This variant has been reported once in ClinVar as Likely pathogenic and once as uncertain significance (Variation ID: 632813). This variant is not present in gnomAD (PM2_Sup; https://gnomad.broadinstitute.org/ v4.0.0). 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). The constraint z-score for missense variants affecting FBN1 is 8.18 (PP2; https://gnomad.broadinstitute.org/ v4.0.0). Computational prediction tools and conservation analysis suggest that this variant may impact the protein (REVEL: 0.992, PP3). 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_Strong, PM2_Sup,PP3, PP2.