NM_000138.5 c.6419G>A is a missense variant in FBN1 predicted to cause a substitution of a cysteine by tyrosine at amino acid 2164 (p.Cys2164Tyr). This variant has been identified in at least four individuals meeting clinical diagnostic criteria for Marfan syndrome (MFS) and two individuals with non-specific features suggestive of a connective tissue disorder (PS4, PP4; Bichat & UZG internal data; PMIDs: 31730815, 37558401, 29095814). In one of these individuals, it was found to be de novo; however, phenotype details were not provided and therefore the specificity and consistency of the phenotype with FBN1 cannot be assessed (PMID: 29095814). In another of these families, the variant was found to segregate with disease in two affected family members (PP1; Bichat internal data). It is absent from gnomAD (PM2_supporting; https://gnomad.broadinstitute.org/, v2.1.1, 3.1.2, 4.0.0). This variant affects a cysteine residue in a calcium-binding EGF-like domain; cysteine residues are involved in the formation of disulfide bridges which are essential for the protein structure (PM1_strong). Computational prediction tools and conservation analysis support that this variant is likely to impact the protein (PP3; REVEL = 0.863). The constraint z-score for missense variants affecting FBN1 is 8.18 (PP2; gnomAD v4.0.0). 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, PP1, PP2, PP3, PP4, PM2_supporting.