NM_00138 c.1453C>T is a missense variant in FBN1 predicted to cause a substitution of an arginine by cysteine at amino acid 485 (p.Arg484Cys). This variant has been identified in the literature in at least seven probands with features consistent with or suggestive of Marfan syndrome including thoracic aortic aneurysm and dissection (TAAD) with or without additional features (PS4; PMIDs: 16342915, 19293843, 24793577, 30485715). It was found in the homozygous state in two cousins with bilateral ectopia lentis (EL) and additional features of Marfan syndrome; of note, in both instances the variant was inherited from mildly affected or apparently unaffected heterozygous parents (PMID: 17568394). It was identified in trans with a different FBN1 pathogenic variant in three siblings with isolated EL (Bichat internal data); clinical information for the heterozygous parents was not available. The variant has also been identified in the heterozygous state in 5 additional internal probands diagnosed with TAAD with or without systemic features (UZG, Johns Hopkins University, Bichat, & University of Tokyo internal data). It has been found to segregate in the heterozygous state with features of Marfan syndrome in at least 19 individuals among five different families (PP1_strong; PMID: 30485715; Johns Hopkins & Bichat internal data). It has been identified as de novo once with confirmed maternity/paternity in an internal proband with a non-specific phenotype (PS2_supporting; PMID: 19293843; UZG internal data). It is not present in gnomAD (PM2_supporting; https://gnomad.broadinstitute.org/). This variant introduces a novel cysteine residue which may impede the normal formation of critical disulfide bridges (PM1). Computational prediction tools and conservation analysis are unclear about the variant’s predicted impact on the protein’s structure or function. 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: PS4, PP1_strong, PM1, PS2_supporting, PM2_supporting, PP2.