The extracellular antagonists of the Wnt signalling pathway can be divided into two broad classes. Both classes of molecule prevent ligand-receptor interactions, but by different mechanisms: members of the first class, which include the sFRP (secreted Frizzled-related protein) family, WIF (Wnt inhibitory factor)-1 and Cerberus, primarily bind to Wnt proteins; the second class comprises certain members of the Dickkopf (Dkk) family, which bind to one subunit of the Wnt receptor complex. In addition, there are other protein interactions that contribute to Wnt antagonist function. Moreover, certain sFRPs and Dkks do not antagonise Wnt function, which suggests that these families have as-yet-undiscovered functions.