Fig. 2. Nuclear Tcf/ß-catenin complexes. (A) In the absence of Wnt stimulation, Tcf binds to the WRE (Wnt responsive element) but recruits Groucho-HDAC to repress Wnt responsive genes. CtBP is involved in this repression but acts in parallel to Tcf (Fang et al., 2006). (B) On Wnt stimulation, stabilized ß-catenin complexes with Tcf and recruits multiple co-activator complexes for transcriptional activation (Mosimann et al., 2006; Sierra et al., 2006). (C) When wild-type APC is expressed in cancer cells that harbor a mutant APC, wild-type APC directly inhibits Myc expression (Sierra et al., 2006) by recruiting CtBP to the Myc promoter (Sierra et al., 2006). A transcriptional repressor, YY1, and the E3 ubiquitin ligase subunit ß-Trcp, which is required for ß-catenin degradation, are also recruited to the Myc promoter (Sierra et al., 2006). The mechanism by which many of these events occur remains unknown. APC may also act together with CtBP to remove ß-catenin from Tcf (Hamada and Bienz, 2004).