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.