NM_00138 c.3689T>C is a missense variant in FBN1 predicted to cause a substitution of a Methionine by Threonine at amino acid 1230 p.(Met1230Thr). This variant was identified in an internal proband with ectopia lentis and thoracic aortic aneurysm and dissection, however a pathogenic frameshift variant in FBN1 was also identified in the proband (BP2) and was considered to explain the phenotype. Furthermore this variant does not segregate with disease in an affected family member (BS4). . This variant has been previously reported in ClinVar as of uncertain significance (Variation ID: 263789). This variant has been identified in 0.0018% of individuals of European origin (https://gnomad.broadinstitute.org/). Computational prediction tools and conservation analysis are unclear on the predicted impact on the protein. The constraint z-score for missense variants affecting FBN1 is 5.06 however due to the presence of one benign argument PP2 cannot be used . In summary, this variant meets criteria to be classified as likely benign for Marfan syndrome based on the ACMG/AMP criteria applied, as specified by the ClinGen FBN1 VCEP: BS4, BP2