Inhibition of myostatin (MSTN) and activin signalling by the soluble activin type IIB receptor (ActRIB). MSTN and activin signal to target cells by binding initially to the two activin type II receptors, ActRIIA and/or ActRIIB (also called Acvr2 and Acvr2b, respectively) and then to the type I receptors, ALK4 and/or ALK5. The activated type I receptors phosphorylate the intracellular mediators of signalling, Smad2 and/or Smad3. Signalling through this pathway results in the inhibition of muscle differentiation and growth. The activities of MSTN and activin are regulated normally by a number of different extracellular binding proteins, such as follistatin and FSTL-3. The soluble form of ActRIIB (ActRIIB/Fc) can act as a ligand trap by binding MSTN and activin and preventing the ligands from binding to their true receptors.