Constitutive activation of the Wnt signaling pathway is a root cause of many colon cancers^,,. Activation of this pathway is caused by genetic mutations that stabilize the β-catenin protein, allowing it to accumulate in the nucleus and form complexes with any member of the lymphoid enhancer factor (LEF1) and T-cell factor (TCF1, TCF3, TCF4) family of transcription factors (referred to collectively as LEF/TCFs) to activate transcription of target genes^,. Target genes such as MYC, CCND1, MMP7 and TCF7 (refs. –) are normally expressed in colon tissue, so it has been proposed that abnormal expression levels or patterns imposed by β-catenin/TCF complexes have a role in tumor progression. We report here that LEF1 is a new type of target gene ectopically activated in colon cancer. The pattern of this ectopic expression is unusual because it derives from selective activation of a promoter for a full-length LEF1 isoform that binds β-catenin, but not a second, intronic promoter that drives expression of a dominant-negative isoform. β-catenin/TCF complexes can activate the promoter for full-length LEF1, indicating that in cancer high levels of these complexes misregulate transcription to favor a positive feedback loop for Wnt signaling by inducing selective expression of full-length, β-catenin–sensitive forms of LEF/TCFs.