SMAD4 and hereditary hemorrhagic telangiectasia: The combined phenotype occurs because SMAD4 mediates signalling through the BMPR1A receptor implicated in juvenile polyposis (JP, OMIM174900), and ACVRL1/Endoglin signalling involved in Hereditary Haemorrhagic Telangiectasia (HHT, OMIM187300 &600376).