Mutations in the endoglin (ENG) and the activin receptor-like kinase 1 (ACVRL1 also known as ALK1) gene cause type 1 and type 2 HHT (HHT1 and HHT2), respectively [58, 59], which are responsible for approximately 90% of all HHT cases [60]. This evidence concerns the gene ACVRL1 and hereditary hemorrhagic telangiectasia.