Aggressive arterial aneurysms, such as thoracic aortic aneurysms and aortic dissection, were found to be caused by mutations in the genes encoding the transforming growth factor β (TGF-β) receptor I or II, which are characteristic of the Loeys–Dietz syndrome. Screening for these mutations in persons at risk may allow preventive measures to be taken. Aggressive arterial aneurysms were found to be caused by mutations in the genes encoding the transforming growth factor β receptor I or II. Screening for these mutations in persons at risk may allow preventive measures to be taken. Mutations in the genes encoding transforming growth factor β (TGF-β) receptors 1 and 2 ( TGFBR1 and TGFBR2, respectively) have recently been found in association with a continuum of clinical features. On the mild end, the mutations have been found in association with a presentation similar to that of Marfan's syndrome or with familial thoracic aortic aneurysm and dissection, 1 , 2 and on the severe end, they are associated with a complex phenotype in which aortic dissection or rupture commonly occurs in childhood. 3 This complex phenotype is characterized by the triad of widely spaced eyes (hypertelorism); a bifid uvula, cleft palate, . . .