Hereditary hemorrhagic telangiectasia (HHT) or Osler-Weber-Rendu syndrome is a hereditary disease that is transmitted in an autosomal dominant manner and is caused by loss-of-function (LOF) mutations in certain components of the predominantly endothelial BMP9-10/ENG/ALK1/SMAD4 signaling pathway, which is an important mediator of vascular quiescence (1). Here, SMAD4 is linked to hereditary hemorrhagic telangiectasia.