ACVRL1 and hereditary hemorrhagic telangiectasia: It was found that mutations in three genes cause HHT, including endoglin (ENG, chromosomal locus 9q34) [7], activin-receptor like kinase 1 (ACVRL1, also known as ALK1, chromosomal locus 12q1) [8] and mothers against decapentaplegic homolog 4 (SMAD4, chromosomal locus 18q21) [9].