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). The gene discussed is ACVRL1; the disease is hereditary hemorrhagic telangiectasia.