A number of serine/threonine kinase receptors have recently been identified. Most of the members of this family are type I or type II receptors for proteins in the transforming growth factor-beta (TGF-beta) superfamily. The type I and type II receptors form heteromeric receptor complexes after binding of the ligands, and transduce intracellular signals. TGF-beta type I receptor needs type II receptor for ligand binding, and type II receptor needs type I receptor for signalling. The signal transducing molecules that interact with heteromeric serine/threonine kinase receptor complexes remain to be elucidated; cyclin-dependent kinases and the retinoblastoma protein appear to be downstream elements in the antiproliferative pathway of TGF-beta.