Transcriptional regulator involved in the initiation of neuronal differentiation and mesenchymal to epithelial transition (PubMed:15226298, PubMed:18214987). 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 (PubMed:18214987). Together with TCF15, required for the mesenchymal to epithelial transition (PubMed:11309385, PubMed:15226298). Dimers bind DNA on E-box motifs: 5'-CANNTG-3' (PubMed:15226298, PubMed:18214987). Binds to the kappa-E2 site in the kappa immunoglobulin gene enhancer (By similarity). Binds to IEB1 and IEB2, which are short DNA sequences in the insulin gene transcription control region (PubMed:2181401). Facilitates ATOH7 binding to DNA at the consensus sequence 5'-CAGGTG-3', and positively regulates transcriptional activity. Homodimer (PubMed:12196028). Heterodimer; efficient DNA binding requires dimerization with another bHLH protein (PubMed:15226298). Forms a heterodimer with TWIST1 and TWIST2 (PubMed:7589808, PubMed:23395635). Forms a heterodimer with NEUROD1; the heterodimer is inhibited in presence of ID2, but not NR0B2, to E-box element (PubMed:23395635, PubMed:18069799). Forms a heterodimer with TCF15; the heterodimer binds E-box element (PubMed:15226298, PubMed:23395635). Forms a heterodimer with MYOG; heterodimerization enhances MYOG DNA-binding and transcriptional activities (PubMed:12196028). Forms a heterodimer with ATOH8; repress transcription of TCF3 and TCF3-NEUROG3 dimer-induced transactivation of E box-dependent promoters (PubMed:23938248). Component of a nuclear TAL-1 complex composed at least of CBFA2T3, LDB1, TAL1 and TCF3 (PubMed:16407974). Interacts with NEUROD2 (PubMed:18214987). Interacts with EP300 (By similarity). Interacts with PTF1A, TGFB1I1 and UBE2I (PubMed:9409784, PubMed:11318877, PubMed:16291758). Interacts with BHLHA9 (By similarity). Interacts with ASB2; the interaction is mediated by SKP2 and targets TCF3 for Notch-induced proteasomal degradation (By similarity). Forms a heterodimer with ATOH7; required for ATOH7 DNA-binding. Interacts with RALGAPA1 (PubMed:12200424). Interacts with FIGLA (By similarity). Expressed during the development of the nervous system. 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. Mice are smaller than their wild-type littermates and fail to thrive 14 days after birth.