Answer:
Marfan syndrome is caused by a mutation in FBN1, which is fibrillin. There are over 600 known mutations that cause Marfan. It's interesting...most mutations are found in an individual family and no one else in the world. Only 12% of mutations overlap between unrelated people. Because of this, genetic testing is not beneficial unless a person 1) already meets many clinical criteria or 2) has an immediate family member who has tested positive for an FBN1 mutation (so that you know whether it's a disorder causing mutation or a benign mutation, like every person has thousands of). Marfan is autosomal dominant, so if a person has the mutation they have the disorder, and they have a 50% chance of passing it on to their offspring.
There are other disorders that also are caused by FBN1 mutations, such as ectopia lentis and MASS Phenotype. Therefore, clinical criteria is still vital to make a firm diagnosis.