The Transforming Growth Factor-β Receptor (TGF-βR) family are transmembrane serine/threonine kinases. They form heterodimeric receptor complexes composed of type I and type II subunits. Upon ligand binding, the constitutively active type II receptor phosphorylates the type I receptor which, in turn, phosphorylates a member of the Smad family. (Adapted from
FBrf0236482.)