NM_000138.5 c.4330T>A is a missense variant in FBN1 predicted to cause a substitution of a cysteine by serine at amino acid 1444 (p.Cys1444Ser). This variant has been identified in an adult patient with Marfan syndrome with bilateral ectopia lentis (EL), borderline thoracic aortic aneurysm and dissection (TAAD), and systemic features (PP4; Invitae internal data). A different nucleotide substitution (c.4331G>C) resulting in the same amino acid change has been identified in three individuals with clinical diagnoses of Marfan syndrome, all of whom were noted to have have EL in addition to other features of Marfan syndrome (PS4_moderate; University of Tokyo, Bichat, & Invitae internal data); in one family, c.4331G>C was found to segregate with annuloaortic ectasia and arachnodactyly in an infant child (University of Tokyo internal data). Of note, there is no predicted difference in splicing patterns between the two nucleotide substitutions. The c.4330T>A variant is absent from gnomAD (PM2_supporting; https://gnomad.broadinstitute.org, v2.1.1 & v3.1.2). It affects a cysteine residue in a calcium-binding EGF-like domain; cysteine residues are involved in the formation of disulfide bridges which are essential for the protein structure (PM1_strong). Computational prediction tools and conservation analysis further support that this variant is likely to impact the protein (PP3; REVEL = 0.949). The constraint z-score for missense variants affecting FBN1 is 5.06 (PP2). In summary, c.4330T>A (p.Cys1444Ser) meets criteria to be classified as pathogenic for Marfan syndrome based on the ACMG/AMP criteria applied, as specified by the ClinGen FBN1 VCEP: PM1_strong, PS4_moderate, PP2, PP3, PP4, PM2_supporting.