The first genes identified as mutated in HHT patients, were Endoglin (ENG), located on chromosome 9q33-34, causing HHT1 (Fernández-Ruiz et al., 1993; McAllister et al., 1994) and ACVRL1/ALK1 (activin receptor-like kinase 1), that causes HHT2 (Johnson et al., 1995, 1996). Here, ENG is linked to hereditary hemorrhagic telangiectasia.