NM_000138.5 c.4526A>G is a missense variant in FBN1 predicted to cause a substitution of a tyrosine by cysteine at amino acid 1509 (p.Tyr1509Cys). This variant has been identified in at least four individuals including two probands with clinical diagnoses of Marfan syndrome, an individual with mild thoracic aortic aneurysm and dissection (TAAD) and systemic features without fulfilling Marfan syndrome diagnostic criteria, and an individual of tall stature with TAAD (PS4_moderate; PMID: 26787436; Johns Hopkins & University of Tokyo internal data). It is absent from gnomAD (PM2_supporting; https://gnomad.broadinstitute.org/ v2.1.1 & v3.1.2). This variant introduces a novel cysteine residue which may impede the normal formation of essential disulfide bridges and occurs at a residue in a critical calcium-binding site within a calcium-binding EGF-like domain (PM1). Computational prediction tools and conservation analysis support that this variant is likely to impact the protein (PP3; REVEL = 0.916). 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_moderate, PP2, PP3, PM2_supporting.