Fig. 8. A model describing the activation of Chordin. Briefly, signalling by Xenopus Nodal-related proteins such as Xnr1 activates Smad2 and Smad3. Smad3 induces expression of Xlim1 directly and Xlim1 and Smicl, perhaps in a ternary complex with Smad3 (indicated by `?'), then induce expression of Chordin.