SMAD5 is a member of the SMAD family and mediates signaling by the transforming growth factor-beta (TGFβ) superfamily and related ligands. SMAD5 plays a critical role in the signaling pathway by which TGFβ inhibits the proliferation of human hematopoietic progenitor cells. SMAD5 is up-regulated in gastric epithelial cells during the infection of the pathogen Helicobacter pylori and it mediates apoptosis of gastric epithelial cells induced by H. pylori infection. In mature human B cells, bone morphogenetic protein 6 (BMP-6) inhibits cell growth and rapidly induces phosphorylation of SMADs 5 and 8.