ACVRL1 and hereditary hemorrhagic telangiectasia: Hereditary hemorrhagic telangiectasia (HHT, Osler–Weber–Rendu syndrome) is an autosomal-dominant vascular disorder caused by heterozygous loss-of-function mutations in activin receptor-like kinase 1 (ALK1), endoglin (ENG), or SMAD4 [5, 6].