The NM_00138 c.2660G>A, is a missense variant in FBN1 predicted to cause a substitution of a cysteine by tyrosine at amino acid 887 (p.Cys887Tyr). This variant was found in a proband with a clinical diagnosis of Marfan syndrome (PMID 23684891) (PP4). This variant has been reported three times in ClinVar: twice as pathogenic and once as uncertain significance (Variation ID: 915814). This variant has been reported in the literature in an individual with a suspected diagnosis of Marfan syndrome (PMID 29907982). This variant is not present in gnomAD (PM2_sup; https://gnomad.broadinstitute.org/ v2.1.1). Other missense variants affecting the same residue, including p.Cys887Tyr and p.Cys887Arg have been previously reported in individuals with clinical features of Marfan syndrome (PMID 27906200, 34281902, internal data; PM5). This variant affects a cysteine residue in a hybrid domain. Cysteine residues are believed to be involved in the formation of disulfide bridges which are essential for the protein structure (PM1). Computational prediction tools and conservation analysis suggest that this variant may impact the protein (REVEL: 0.926, 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, PM5, PM2_Sup, PP2, PP3, PP4.