NM_000138.5 c.4280A>G is a missense variant in FBN1 predicted to cause a substitution of a tyrosine by cysteine at amino acid 1427 (p.Tyr1427Cys). This variant has been identified in the literature in an individual with either a suspicion or diagnosis of Marfan syndrome, and by a commercial laboratory in two unrelated individuals with thoracic aortic aneurysm and dissection (TAAD), segregating with TAAD in one similarly affected family member (PS4_supporting; PMID: 21907952, Invitae internal data). It was also identified by a commercial laboratory in an individual with Marfan syndrome including ectopia lentis and a systemic score of 8 (PP4; GeneDx internal data). It is absent from gnomAD (PM2_supporting; https://gnomad.broadinstitute.org/ v2.1.1 & 3.1.2). 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 = 0.939). Another variant affecting the same residue (p.Tyr1427Asp) is classified as likely pathogenic, further supporting the functional importance of this amino acid position (PM5; PMID: 17657824). 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, PS4_supporting, PP2, PP3, PP4, PM2_supporting.