Initial investigations into the etiology of HHT-associated vascular malformations focused on the expression of endoglin (encoded by ENG) and ALK1 (the commonly used term for the protein encoded by ACVRL1) in monocytes and neonatal human umbilical vein endothelial cells (HUVECs) from affected individuals [16,17,18,19,20]. This evidence concerns the gene ACVRL1 and hereditary hemorrhagic telangiectasia.