TCF1, LEF1, TCF3 and TCF4 are HMG box-containing DNA-binding proteins that recognize WNT-responsive elements (WREs) in the promoters of WNT target genes (reviewed in Brantjes et al, 2002). In the absence of a WNT signal, promoter-bound TCF/LEF is bound by one of four Groucho homologues, TLE1, 2, 3 or 4 (Levanon et al, 1998; Brantjes et al, 2001; Daniels and Weis, 2005). Groucho/TLE proteins are co-repressors for a variety of DNA-binding transcription factors and mediate repression at least in part through their interaction with histone deacetylases such as RPD3/HDAC1 (Arce et al, 2009; Brantjes et al, 2001; Chen et al, 1999; reviewed in Chen and Courey, 2000). Groucho proteins have been shown to homo-tetramerize through a glutamine rich Q domain at the N-terminus, and this oligomerization is required for repression. The Q domain is also sufficient for interaction with TCF/LEF proteins (Brantjes et al, 2001; Chen et al, 1998; Pinto and Lobe, 1996; Song et al, 2004). Studies with purified proteins have shown that human TLE1 and 2 bind to an amino-terminal truncated form of LEF1(69-397) with an affinity comparable to that for full length LEF1 (Daniels and Weis, 2005).