GDF2 and hereditary hemorrhagic telangiectasia: Additionally, mutations in the SMAD family member 4 (SMAD4) gene cause a combined syndrome of HHT and juvenile polyposis [9] accounting for ~ 2% of the HHT cases and, recently, mutations in the GDF2 gene on chromosome 10, coding for bone morphogenetic protein 9 (BMP9), has been implicated as a cause of HHT (HHT5) [8].