Transcriptional regulator involved in the initiation of neuronal differentiation and mesenchymal to epithelial transition (By similarity). Heterodimers between TCF3 and tissue-specific basic helix-loop-helix (bHLH) proteins play major roles in determining tissue-specific cell fate during embryogenesis, like muscle or early B-cell differentiation (By similarity). Together with TCF15, required for the mesenchymal to epithelial transition (By similarity). Dimers bind DNA on E-box motifs: 5'-CANNTG-3' (By similarity). Binds to the kappa-E2 site in the kappa immunoglobulin gene enhancer (PubMed:2493990). Binds to IEB1 and IEB2, which are short DNA sequences in the insulin gene transcription control region (By similarity). Facilitates ATOH7 binding to DNA at the consensus sequence 5'-CAGGTG-3', and positively regulates transcriptional activity. Homodimer (PubMed:2112746, PubMed:14752053). Heterodimer; efficient DNA binding requires dimerization with another bHLH protein (By similarity). Forms a heterodimer with ASH1, TWIST1 and TWIST2 (By similarity). Forms a heterodimer with MYOG; heterodimerization enhances MYOG DNA-binding and transcriptional activities (By similarity). Forms a heterodimer with NEUROD1; the heterodimer is inhibited in presence of ID2, but not NR0B2, to E-box element (PubMed:14752053). Forms a heterodimer with TCF15; the heterodimer binds E-box element (By similarity). Forms a heterodimer with ATOH8; repress transcription of TCF3 and TCF3/NEUROG3 dimer-induced transactivation of E box-dependent promoters (By similarity). Component of a nuclear TAL-1 complex composed at least of CBFA2T3, LDB1, TAL1 and TCF3 (By similarity). Interacts with NEUROD2, PTF1A and TGFB1I1 (By similarity). Interacts with EP300 and UBE2I (PubMed:9409784, PubMed:14752053). Interacts with BHLHA9 (PubMed:25466284). Interacts with ASB2; the interaction is mediated by SKP2 and targets TCF3 for Notch-induced proteasomal degradation (PubMed:21119685). Forms a heterodimer with ATOH7; required for ATOH7 DNA-binding. Interacts with RALGAPA1 and FIGLA. the 9aaTAD motif is a transactivation domain present in a large number of yeast and animal transcription factors. Phosphorylated following NGF stimulation. Undergoes Notch-induced ubiquitination and subsequent proteasomal degradation which is mediated by ASB1 or ASB2, the substrate-recognition components of probable ECS E3 ubiquitin-protein ligase complexes. Chromosomal aberrations involving TCF3 are cause of forms of pre-B-cell acute lymphoblastic leukemia (B-ALL). Translocation t(1;19)(q23;p13.3) with PBX1. TCF3-PBX1 transforms cells by constitutively activating transcription of genes regulated by PBX1 or by other members of the PBX protein family (PubMed:1967983, PubMed:1671560). Translocation t(17;19)(q22;p13.3) with HLF (PubMed:1386162). Inversion inv(19)(p13;q13) with TFPT (PubMed:10086727). The disease is caused by variants affecting the gene represented in this entry. The bHLH domain encompassing amino acids 546 to 599 is sufficient to mediate DNA-binding and homodimerization. Combined mutagenesis of Phe-566 and Leu-569 to Asp-566 and Glu-569, mutagenesis of Lys-585 to Ala-585 or combined mutagenesis of Ile-588 and Leu-589 to Asp-588 and Glu-589 prevents DNA-binding and homodimerization. Mutagenesis of Arg-548 to Lys-548, combined mutagenesis of Arg-547 and Arg-548 to Gly-547 and Gly-548, mutagenesis of Arg-556 to Lys-556, mutagenesis of Arg-558 to Lys-558, or combined mutagenesis of Arg-556 and Arg-558 to Gly-556 and Gly-558, alter DNA-binding but not dimerization. Truncated N-terminus.