Mutations in ENG (encoding Endoglin), ACVRL1 (encoding ALK1) and SMAD4 cause HHT1 (MIM: 187300), HHT2 (MIM: 600376), and the combined Juvenile Polyposis/HHT (MIM: 175050) syndrome, respectively 44. Here, ENG is linked to hereditary hemorrhagic telangiectasia.