ACVRL1 and hereditary hemorrhagic telangiectasia: HHT is genetically heterogeneous with heterozygous mutations in at least three known genes including endoglin (ENG) causing HHT1 [6], activin receptor-like kinase 1 (ACVRL1 or ALK1), causing HHT2 [7], and mothers against decapentaplegic homolog 4 (MADH4 or SMAD4) causing familial juvenile polyposis associated with HHT (JP/HT) [8].