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 (By similarity). Binds to the consensus sequence CAC/GCTGT/C present, in the chymotrypsin, insulin, AP-4, and several other gene enhancer motifs (PubMed:2200736). Facilitates ATOH7 binding to DNA at the consensus sequence 5'-CAGGTG-3', and positively regulates transcriptional activity. Homodimer. Heterodimer; efficient DNA binding requires dimerization with another bHLH protein. Forms a heterodimer with TWIST1 and TWIST2. Forms a heterodimer with NEUROD1; the heterodimer is inhibited in presence of ID2, but not NR0B2, to E-box element. Forms a heterodimer with TCF15; the heterodimer binds E-box element. Forms a heterodimer with MYOG; heterodimerization enhances MYOG DNA-binding and transcriptional activities. Forms a heterodimer with ATOH8; repress transcription of TCF3 and TCF3-NEUROG3 dimer-induced transactivation of E box-dependent promoters. Component of a nuclear TAL-1 complex composed at least of CBFA2T3, LDB1, TAL1 and TCF3. Interacts with NEUROD2 (By similarity). Interacts with EP300 (By similarity). Interacts with PTF1A, TGFB1I1 (By similarity). Interacts with UBE2I (PubMed:9013644). Interacts with BHLHA9 (By similarity). Interacts with ASB2; the interaction is mediated by SKP2 and targets TCF3 for Notch-induced proteasomal degradation (By similarity). Interacts with RALGAPA1 (By similarity). Interacts with FIGLA (By similarity). Forms a heterodimer with ATOH7; required for ATOH7 DNA-binding. 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.