Similarly, it is not known to what extent loss of the antiproliferative role of BMP9 signalling in the absence of ENG or ACVRL1 proteins drives the increased EC proliferation seen in telangiectases from HHT2 patients and in AVMs in preclinical models of HHT [17,44,45,46,47]. This evidence concerns the gene ACVRL1 and hereditary hemorrhagic telangiectasia.