The TGF-β family comprises many structurally related differentiation factors that act through
a heteromeric receptor complex at the cell surface and an intracellular signal transducing
Smad complex. The receptor complex consists of two type II and two type I transmembrane
serine/threonine kinases. Upon phosphorylation by the receptors, Smad complexes
translocate into the nucleus, where they cooperate with sequence-specific transcription
factors to regulate gene expression. The vertebrate genome encodes many ligands, fewer …