The NM_000138.5 c.6453C>T variant in FBN1does not result in an amino acid substitution. This variant was found in a proband with thoracic aortic aneurysm and/or dissection, and a systemic score of 8, which is a highly specific phenotype for Marfan syndrome (internal data-Ghent university Hospital) (PP4). This variant has been reported 6 times in ClinVar: 4 times as pathogenic, 2 times as likely pathogenic, and once as variant of uncertain significance (Variation ID: 200193). The variant has been reported in the literature in at least two unrelated individuals with a clinical diagnosis of Marfan syndrome, and in at least four unrelated individuals with clinical features of Marfan syndrome (PMID 26787436, 36517271, 21907952, 32679894, 36517271, InvitaeClinVarentry) (PS4). This variant was also found to segregate with disease in at least 5 affected relatives from multiple families (PMID 21907952, internal data) (PP1_Strong). In silico prediction programs predict that this variant may impact splicing (PP3). RNA functional studies confirmed that this variant results in the creation of a cryptic donor splice site resulting in an abnormal truncated protein (PMID 21907952, 26787436, 32123317) (PS3). This variant is not present in gnomAD (PM2_sup; https://gnomad.broadinstitute.org/ version 2.1.1). 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: PS3, PS4, PP1_Strong, PM2_Sup, PP3, PP4