FBN1 and Marfan syndrome: While a monogenic etiology is characteristic of Marfan syndrome (mutation in the FBN1 gene encoding the fibrillin-1 protein) or Loeys-Dietz syndrome (mutations in TGFBR1, TGFBR2, and other genes involved in the transforming growth factor β (TGF-β) pathway), degenerative changes in the aorta are observed in older people.