GDF2 and hereditary hemorrhagic telangiectasia: The HHT-causing mutations affect genes encoding different components of the BMP9/10 pathway; namely, ENG encoding the membrane glycoprotein ENDOGLIN in HHT1, ACVRL1 encoding the membrane receptor ALK1 in HHT2, SMAD4 encoding the intracellular signaling molecule SMAD4 in Juvenile polyposis (JP)-HHT, and GDF2 encoding BMP9 ligand in HHT5 [78–82].