Endoglin (ENG, chromosome 9q34), activin A receptor type II-like 1 (ACVRL1/ALK1, chromosome 12q13), and SMAD4 (chromosome 18q21) mutations cause HHT1 (OMIM 187300), HHT2 (OMIM 600376), and the combined Juvenile Polyposis/HHT (JP/HHT) syndrome (OMIM 175050), respectively (Shovlin et al., 1994; Johnson et al., 1996; Gallione et al., 2004). Here, ACVRL1 is linked to hereditary hemorrhagic telangiectasia.