When Sirolimus was given following liver transplantation to a patient with HHT who had multiple arteriovenous malformations, internal and external telangiectasia, epistaxis, and anemia disappeared, suggesting that the mechanism of action of sirolimus involved partial correction of endoglin and ALK1 haploinsufficiency [67]. Here, ACVRL1 is linked to hereditary hemorrhagic telangiectasia.