ACVRL1 and hereditary hemorrhagic telangiectasia: More than 95% of HHT cases are caused by mutations in transforming growth factor-β (TGF-β)/bone morphogenetic protein (BMP) signalling pathway genes, including the surface receptors Endoglin (ENG, mutated in HHT1) and ACVRL1 (hereafter referred as ALK1, mutated in HHT2), and the signalling pathway effector SMAD4-juvenile polyposis2, 3, 4, 5.