NM_00138 c.1538G>T is a missense variant in FBN1 predicted to cause a substitution of a cysteine by phenylalanine at amino acid 513 (p.Cys513Phe). This variant has not been reported in the literature but has been found in at least two probands; this includes one individual with bilateral ectopia lentis and thoracic aortic aneurysm and dissection, and one with unspecified features of a connective tissue disorder (PS4_supporting; Invitae & LabCorp internal data, ClinVar Variation ID: 406288). This variant is not present in gnomAD (PM2_supporting; https://gnomad.broadinstitute.org/). This variant affects a cysteine residue in a calcium binding EGF domain; cysteine residues are believed to be involved in the formation of disulfide bridges which are essential for the protein structure (PM1_strong). Several other variants affecting this codon have been reported in association with Marfan syndrome (p.Cys513Gly, p.Cys513Ser, p.Cys513Tyr, p.Cys513Arg). Computational prediction tools and conservation analysis strongly support that this variant may impact the protein’s structure or function (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: PM1_strong, PM2_supporting, PS4_supporting, PP2, PP3.