Further underscoring the importance of BMP signaling in vascular homeostasis, mutations in ALK1, GDF2/BMP9, SMAD4, and Endoglin are responsible for the development of hereditary hemorrhagic telangiectasia (HHT), a disease characterized by the formation of arteriovenous malformations throughout the lung, brain, liver, skin, and mucus membranes. Here, SMAD4 is linked to hereditary hemorrhagic telangiectasia.