Heterozygous mutations in at least three genes, including endoglin (ENG, chromosomal locus 9q34), activin-A type-II receptor-like kinase 1 (ACVRL1, also known as ALK1, chromosomal locus 12q1), and mothers against decapentaplegic homolog 4 (SMAD4, chromosomal locus 18q21) are known to cause HHT [6,7,8]. Here, SMAD4 is linked to hereditary hemorrhagic telangiectasia.