Fig. 3. Possible mechanisms by which SFRPs could modulate Wnt signalling. (A) SFRPs could sequester Wnt either through the CRD or NTR domain, thereby acting as classical antagonists. (B) They could titrate one another's activity, thereby favouring Wnt signalling. (C) SFRPs could act in a dominant-negative fashion by forming signalling-inactive complexes with Fz receptors, thereby preventing signal transduction by Wnt. (D) SFRPs might favour a Wnt-Fz interaction by simultaneously binding to both molecules and promoting signal transduction.