The NM_00138 c.1147G>A is a missense variant in FBN1 predicted to cause a substitution of a glutamic acid by lysine at amino acid 383 (p.Glu383Lys). This variant was found in a proband with a systemic score >7 and thoracic aortic aneurysm and/or dissection, which is a highly specific phenotype for Marfan syndrome (MFS) (PMID 11700157, internal lab data, PP4). This variant has been reported four times in ClinVar: once as pathogenic, once as likely pathogenic, and twice as uncertain significance (Variation ID: 200177). This variant is not present in gnomAD (PM2_sup; https://gnomad.broadinstitute.org/ v2.1.1). This variant has been reported in the literature in at least three individuals with clinical features of MFS and/or thoracic aortic aneurysm and dissection (PS4_Mod; PMID 19159394, Invitae ClinVar entry, internal lab data) and in an individual with left iliac aneurysm (GeneDx internal lab data). Computational prediction tools and conservation analysis are inconclusive with regards to a possible impact on this variant’s protein function and structure (REVEL: 0.702). This variant is located in the last nucleotide of exon 9. In silico prediction programs predict that this variant may disrupt RNA splicing (PP3). 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: PS4_Mod, PM2_Sup, PP2, PP3, PP4