In humans, inactivating mutations in the endoglin (ENG) gene are responsible for hereditary hemorrhagic telangiectasia (HHT)1, whereas HHT2 is due to mutations affecting ALK1 (also known as ACVRL1), a type I TGF-β receptor specifically expressed in ECs. The gene discussed is ACVRL1; the disease is telangiectasia, hereditary hemorrhagic, type 2.