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). . [Isoform E47]: Facilitates ATOH7 binding to DNA at the consensus sequence 5'-CAGGTG-3', and positively regulates transcriptional activity. . This is the function of TCF3 (transcription factor 3, ENSG00000071564).