ACVRL1 and hereditary hemorrhagic telangiectasia: Human mutations in the Type 1 receptor Alk1/ACVRL1, the co-receptor Endoglin, or a common signaling component SMAD4 lead to Hereditary Hemmorrhagic Telangiectasia (HHT) that is characterized by arterio-venous malformations (AVMs) and hemorrhage [30–32].