T-cell factor-1 (TCF-1) is a DNA-binding transcriptional activator that isessential for lymphoid cell development. The TCF family of transcription factors are activated by the Wnt-1 and Wingless pathways and are characterized by the presence of a conserved protein motif, the high mobility group (HMG) 1 box, which mediates DNA binding. Several alternative splice variants of TCF-1 have been identified, including TCF-1A, which share a conserved amino terminus and differ in the carboxy terminal sequences. The Wnt mediated signaling pathway induces cytosolic beta-catenin binding to TCF proteins within the nucleus, leading to the enhanced expression of the Wnt target genes. The beta-catenin-TCF complexes are negatively regulated by the adenomatous polyposis coli (APC) tumor suppressor protein, which phosphorylates beta-catenin and, in turn, increases the degradation of cytosolic beta-catenin and inhibits the transcriptional activity of the TCF proteins.