The importance of ALK1 and ENG in the vasculature is underpinned by human genetics showing that autosomal dominant loss-of-function mutations in these two genes cause Hereditary Haemorrhagic Telangiectasia (HHT)9,10, a vascular abnormality characterised by telangiectases (broken capillaries) in the nasal mucosa, gastrointestinal tract and skin, and larger arteriovenous malformations in brain, lungs and liver, which can be life-threatening. Here, ACVRL1 is linked to hereditary hemorrhagic telangiectasia.