TGFβ ligands fall into four functionally distinct subgroups differing in receptor activation and signaling modulation.
A large number of TGFβ/BMP ligands signal though a limited set of receptor.
Numeral ligand–receptor discrepancy leads to promiscuous receptor usage.
Mechanisms have evolved to facilitate not only correct ligand-type I receptor pairing but also strict type I receptor-Smad pairing ensuring activation of only one particular SMAD branch.
Structures of TGFβ/BMP receptor kinases enable the design of specific kinase inhibitors.