The FBN1 gene, which encodes the fibrillin-1 protein, is primarily associated with Marfan syndrome. However, it is not used as the primary diagnostic tool for Marfan syndrome. While mutations in the FBN1 gene are causative in approximately 90% of cases of Marfan syndrome, there is still a significant percentage of cases (around 10%) where no FBN1 mutations can be identified.

The diagnostic process for Marfan syndrome involves a combination of clinical evaluations, family history, and specific diagnostic criteria established by the Ghent criteria. These criteria include evaluations for various physical features, such as skeletal abnormalities, eye problems, and cardiovascular manifestations. The Revised Ghent Criteria, updated in 2010, provide more comprehensive guidelines for diagnosing Marfan syndrome and can help identify affected individuals even in cases where no FBN1 mutation is identified.

Since Marfan syndrome can manifest differently in affected individuals, a combination of clinical assessments, imaging tests, and genetic analysis of multiple genes involved in the condition (including but not limited to FBN1) is necessary for accurate diagnosis. Relying solely on FBN1 gene analysis may miss a considerable portion of cases, leading to potential misdiagnoses or delayed diagnoses.