ACVRL1 and hereditary hemorrhagic telangiectasia: These genes both intervene in the BMP9/ALK1/ENG/SMAD pathway in endothelial cells, and it has been hypothesized that HHT is related to disequilibrium in the angiogenic balance, resulting from an increase in the factors involved in the activation phase and a decrease in those involved in the maturation phase of angiogenesis [2].