SMAD4 and hereditary hemorrhagic telangiectasia: Mutations in genes encoding the co-receptor endoglin (ENG, MIM: 187300), the activin receptor like type 1 (ACVRL1, MIM: 600376) or ALK1 (ALK1), and occasionally also in SMAD4 (SMAD4; MIM: 175050), BMP9 (GDF2; MIM: 615506) and BMPRII (BMPR2; MIM: 600799), cause different subtypes of HHT [221,222,223,224,225].