FBN1 and mitral valve disorder: Moreover, defective fibrillin-1 alters transforming growth factor-ß (TGF-ß) signaling, which in FBN1 deficient mice has been shown to account for manifestations of Marfan syndrome such as pulmonary emphysema, mitral valve prolapse, skeletal muscle myopathy and aortic root dilatation [3]–[6].