Transcriptional regulator (PubMed:1710033). 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 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. 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 and UBE2I (By similarity). 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.