The NM_00138 c.3677G>T is a missense variant in FBN1 predicted to cause a substitution of a glycine by valine at amino acid 1226 (p.Gly1226Val), in a cbEGF-like domain of the protein. This variant was found in a pediatric proband with ectopia lentis and severe annuloaortic ectasia, which is a highly specific phenotype for Marfan syndrome (MFS) (PMID 26796135; PP4). This variant has been reported twice in ClinVar: once as likely pathogenic and once as uncertain significance (Variation ID: 495598). This variant is not present in gnomAD (PM2_sup; https://gnomad.broadinstitute.org/ v2.1.1). Computational prediction tools and conservation analysis suggest that this variant may impact the protein (REVEL: 0.912). The constraint z-score for missense variants affecting FBN1 is 5.06 (PP2). Due to insufficient evidence, this variant is classified as uncertain significance for Marfan syndrome based on the ACMG/AMP criteria applied, as specified by the ClinGen FBN1 VCEP: PM2_Sup, PP2, PP3, PP4.