The NM_00138 c.5518C>T is a missense variant in FBN1 predicted to cause a substitution of an arginine by cysteine at amino acid 1840 (p.Arg1840Cys) within a calcium binding EGF-like domain of the protein. Cysteine residues in these domains 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 who met revised Ghent criteria, which is a highly specific phenotype for Marfan syndrome (PMID 31149040, PP4). This variant has been reported three times in ClinVar: twice as likely pathogenic and once as uncertain significance (Variation ID: 519783). It has been reported in the literature in individuals with clinical features of Marfan syndrome (PMID 25652356, 29357934, 37042257, Internal data, PS4_Sup). This variant is present in 1/35435 (0.003%) of alleles tested from the Admixed American population gnomAD (https://gnomad.broadinstitute.org/v2.1.1). Computational prediction tools and conservation analysis suggest that this variant may impact the protein (REVEL: 0.78, PP3). 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: PM1, PS4_Sup, PP2, PP3, PP4