TGF-beta inhibits the growth of many cell types, including hematopoietic cells and lymphocytes. TGF-beta transduces signals through two different types of serine/threonine kinase receptors, type I (T beta R-I) and type II (T beta R-II). T beta R-II is a primary binding protein for the ligands, and T beta R-I is an effector protein, which determines the specificity of signals. Type III receptor (betaglycan) and endoglin play more indirect roles; i.e. delivery of ligands to the signaling receptors. Various molecules, including farnesyl transferase-alpha, Mothers against dpp (Mad)-related proteins, and a novel MAPKKK (TAK1), have been suggested to participate in the signal transduction of TGF-beta receptors. TGF-beta receptors and Mad-related proteins have been found to act as tumor suppressor genes in various tumors, including colorectal cancers and T-cell lymphoma.