Transcriptional activator involved in T-cell lymphocyte differentiation. Necessary for the survival of CD4(+) CD8(+) immature thymocytes. Isoforms lacking the N-terminal CTNNB1 binding domain cannot fulfill this role. Binds to the T-lymphocyte-specific enhancer element (5'-WWCAAAG-3') found in the promoter of the CD3E gene. Represses expression of the T-cell receptor gamma gene in alpha-beta T-cell lineages (PubMed:17218525). Inhibits the developmental program of IL17A effector gamma-delta T-cell subsets via regulating the transcription of T-cell lineage effector proteins (PubMed:23562159, PubMed:30413363). Required for the development of natural killer receptor-positive lymphoid tissue inducer T-cells (PubMed:23562159). TLE1, TLE2, TLE3 and TLE4 repress transactivation mediated by TCF7 and CTNNB1 (By similarity). May also act as feedback transcriptional repressor of CTNNB1 and TCF7L2 target genes. Binds the armadillo repeat of CTNNB1 and forms a stable complex (By similarity). Binds TLE5, TLE1, TLE2, TLE3 and TLE4 (By similarity). Interacts with MLLT11 (By similarity). Interacts (via N-terminus) with SOX13; inhibits WNT-mediated transcriptional activity. Additional isoforms seem to exist. T-cell specific. Expressed in triple negative 2 subpopulations of T-cells and both the gamma-delta and alpha-beta T-cell lineages (PubMed:17218525). Expressed in Il7 receptor positive innate-like T-cells in the mesenteric lymph nodes and spleen (at protein level) (PubMed:23562159). Expressed in the yolk sac. Defects in Tcf7 may allow the formation of epithelial tumors. Aberrant expression of Vgamma2 T-cell receptor in alpha-beta T-cells (PubMed:17218525). Reduced frequency of DN1d cells, a subset of DN1 precursor thymocytes (PubMed:30413363). Decreased number of natural killer receptor-positive lymphoid tissue inducer T-cells (PubMed:23562159). Increased expression of Il17 in Vgamma1.1 and Vgamma2 gamma-delta T-cells (PubMed:23562159). Loss of Lef1 expression in Vgamma1.1 and Vgamma2 gamma-delta T-cells (PubMed:23562159). Loss of Cd27 expression in Il17a expressing gamma-delta T-cells (PubMed:23562159). Double knockout of Sox13 and Tcf7/Tcf1 show a reduced number of DN1d cells (PubMed:30413363). Belongs to the TCF/LEF family.