The NM_00138 c.5966G>T, is a missense variant in FBN1 predicted to cause a substitution of a cysteine by phenylalanine at amino acid 1989 (p.Cys1989Phe). This variant was found in a proband diagnosed with typical Marfan syndrome (internal lab data, PP4); in this family, the variant was found to be de novo in the proband’s symptomatic mother (internal lab data, PM6). This variant has been reported three times in ClinVar: once as pathogenic, once as likely pathogenic, and once as uncertain significance (Variation ID: 638559). At least three other probands with Marfan syndrome and/or clinical features of Marfan syndrome carry the same variant (PMID 29848614, Petrovsky National Research Centre of Surgery ClinVar entry, Cologne University ClinVar entry, 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 a calcium binding EGF-like domain. Cysteine residues in these domains 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: 0.947, 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_Mod, PM2_Sup, PP2, PP3, PP4.