The identification of Nrp1 as a modulator of Alk1 signalling output, and the striking normalization of the hypervascular phenotype in Alk1-deficient retinas by deletion of a single copy of Nrp1 raises the prospect that targeting Nrp1 activity could provide treatment options for Hereditary Haemorrhagic Telangiectasia (HHT) patients carrying inactivating Alk1 mutations58. Here, NRP1 is linked to hereditary hemorrhagic telangiectasia.