Hereditary hemorrhagic telangiectasia (HHT) is an inherited genetic disorder caused by autosomal dominant mutations in Endoglin (ENG; HHT1), Activin receptor like kinase-1 (ACVRL1; HHT2) or SMAD4 (HHT3), genes that mediate signaling by transforming growth factor β (TGF-β) and bone morphogenetic protein (BMP) in vascular endothelial cells (ECs) (Goumans et al., 2009). The gene discussed is ACVRL1; the disease is hereditary hemorrhagic telangiectasia.