A majority of HHT patients have mutations in either endoglin (ENG, chromosome 9q34) or activin A receptor type II-like 1 (ACVRL1/ALK1, chromosome 12q13), both of which are components of the transmembrane receptor complex for TGF-β pathway ligands [1, 3, 4]. The gene discussed is ENG; the disease is hereditary hemorrhagic telangiectasia.