ACVRL1 and hereditary hemorrhagic telangiectasia: HHT is caused by mutations in ENG (encoding endoglin) [3], ACVRL1 (encoding activin receptor-like kinase 1) [4], or MADH4 (encoding SMAD4), which are responsible for an imbalanced state between anti- and pro-angiogenic factors, such as vascular endothelial growth factor (VEGF) [5].