Physical interaction between the lymphoid high mobility group (HMG)-box architectural transcription factors TCF/LEF and beta-catenin is associated with translocation of the heteromeric complex to the nucleus and regulation of target gene expression. Since formation of molecular complexes among beta-catenin, E-cadherin, p300apc and TCF/LEF depends on balanced expression of these constituents, we investigated the biosynthesis of TCF-1 in colorectal cancer. Here we report detailed analyses of activation and overexpression of lymphoid transcription factor TCF-1 in human colorectal cancer-derived cell lines. Northern blot analyses revealed considerable steady-state expression levels of TCF-1 mRNA of normal size. Genomic rearrangement of the 5' flanking region of the TCF-1 gene was excluded as a cause of ectopic expression. By contrast, CAT-reporter constructs depending on a 515-bp T-cell-regulated TCF-1 genomic upstream region were significantly activated in epithelial tumor cells. RT-PCR analyses revealed a heterogeneic population of mRNA isoforms due to alternative splicing in the TCF-1 gene. On Western blots of colorectal cancer cells, the TCF-1-specific monoclonal antibody 7H3 detected a similar heterogeneous spectrum of TCF-1 specific polypeptide chains. Interestingly, overexpression of TCF-1-specific splice forms correlated with the metastatic behavior of the analyzed cells and with overproduction of lymphoid tyrosine protein kinase p56(lck). We conclude that ectopic expression of the HMG-box factor TCF-1 is associated with late events in tumor progression.