Activin receptor‐like kinase‐1 plays a key role in the development of vessel networks, as demonstrated in type‐2 hereditary hemorrhagic telangiectasia (HHT) (Osler–Weber–Rendu syndrome), which is a disease characterized by loss‐of‐function mutations in the ACVRL1 gene encoding for ALK‐1 and by abnormal vessel development (e.g., vascular dysplasia syndrome and arterial venous malformations) 6, 7, 8. The gene discussed is ACVRL1; the disease is hereditary hemorrhagic telangiectasia.