Mutation in the gene of the protein amillin, that can cause problems like aortic dissection or bilateral luxation of the lens.
Basically the connective tissue is in lack of collagen so this syndrom which is by the way hereditary for its genetic causes touches multiple organism : Bones (pectus excavatum or carinatum/ juvenile osteoprasis/scoliosis /kyphosis lordosis...) joints (ligamentous hyperlaxitis...) skin( hyperlastic skin /stretch mark...) eyes ( retinal detachment) vascular ( aortic dilation) with high risk of pneumonia People with marfan are usually tall and skin