The NM_00138 c.199T>C is a missense variant in FBN1 predicted to cause a substitution of a cysteine by arginine at amino acid 67 (p.Cys67Arg). 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 1 in ClinVar as likely pathogenic and 1 time as uncertain significance (Variantion ID: 1380036). Three other probands with clinical features of Marfan syndrome carry the same variant (PMID 19839986, 33436942, 34281902, PS4_Mod). This variant is not present in gnomAD (PM2_sup; https://gnomad.broadinstitute.org/ v2.1.1). This variant affects a cysteine residue in the fibrillin unique N-terminal (FUN) domain. A different missense variant impacting the same residue, p.Cys67Ser, has previously been established as likely pathogenic, and reported in individuals with clinical features of Marfan syndrome (PMID 27906200, internal data, PM5). Cysteine residues are believed to be involved in the formation of disulfide bridges which are essential for the protein structure. Computational prediction tools and conservation analysis suggest that this variant may impact the protein (REVEL: 0.883, 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: PS4_Mod, PM5, PM2_Sup, PP2, PP3, PP4