In human, heterozygous mutations of ALK1 or endoglin (ENG), a non-kinase accessory protein for ALK1, account for hereditary hemorrhagic telangiectasia (HHT), a familial human vascular syndrome that is characterized by development of fragile and direct connection between arteries and veins, or arteriovenous malformations [6, 7]. Here, ACVRL1 is linked to hereditary hemorrhagic telangiectasia.