Causative mutations in one of several genes, most commonly endoglin (ENG) or activin receptor-like kinase 1 (ACVRL1) and less commonly SMAD4, lead to the characteristic HHT lesions—large vessel arteriovenous malformations (AVMs) and/or small vessel telangiectases [2–4]. This evidence concerns the gene SMAD4 and hereditary hemorrhagic telangiectasia.