In humans deregulated TGF-β signalling by haplo-insufficiency for either endoglin or ALK1 results in arteriovenous malformations that are a hallmark of hereditary haemorrhagic telangiectasia type 1 and type 2 (HHT1 or 2) [19]. The gene discussed is ALK; the disease is arteriovenous hemangioma/malformation.