Mutations in two genes, Endoglin (also designated CD105) and ACVRL1 (activin receptorlike kinase 1, also designated TGF(beta) superfamily RI), are responsible for HHT. Endoglin is mutated in HHT1, and ALK-1 is mutated in HHT2, both of which are thought to be caused by haploinsufficiency. Endoglin and ALK-1 are type III and type I members of the TGF(beta) receptor superfamily, respectively, that are expressed on vascular endothelial cells. Endoglin can only bind ligands of the TGF(beta) superfamily via association with the respective ligand binding receptors for TGF(beta)1, TGF(beta)3, Activin-A, BMP-2 and BMP-7. ALK-1 preferentially binds TGF(beta)1 and is expressed in bone marrow stromal cells, lung, brain, kidney and spleen.