The so-far identified causative genes are ENG and ACVRL1 (accounting for HHT1 and HHT2, respectively, over 85% of all HHT cases), SMAD4 (JP/HHT phenotype, 2% of HHT cases) and GDF2 (HHT5, reported occasionally) [13]. This evidence concerns the gene ACVRL1 and hereditary hemorrhagic telangiectasia.