The WIF domain is found in the RYK tyrosine kinase receptors and WIF the Wnt-inhibitory-factor. The domain is extracellular and contains two conserved cysteines that may form a disulphide bridge. This domain is Wnt binding in WIF, and it has been suggested that RYK may also bind to Wnt. The WIF domain is a member of the immunoglobulin superfamily, and it comprises nine beta-strands and two alpha-helices, with two of the beta-strands (6 and 9) interrupted by four and six residues of irregular secondary structure, respectively. Considering that the activity of Wnts depends on the presence of a palmitoylated cysteine residue in their amino-terminal polypeptide segment, Wnt proteins are lipid-modified and can act as stem cell growth factors, it is likely that the WIF domain recognizes and binds to Wnts that have been activated by palmitoylation and that the recognition of palmitoylated Wnts by WIF-1 is effected by its WIF domain rather than by its EGF domains. A strong binding affinity for palmitoylated cysteine residues would further explain the remarkably high affinity of human WIF-1 not only for mammalian Wnts, but also for Wnts from Xenopus and Drosophila.