SMAD4 and hereditary hemorrhagic telangiectasia: HHT is caused by heterozygous mutations in at least three known genes: endoglin (ENG, chromosomal locus 9q34) [6], activin receptor-like kinase 1 (ACVRL1, also known as ALK1, chromosomal locus 12q1) [7] and mothers against decapentaplegic homolog 4 (SMAD4, chromosomal locus 18q21) [8].