Transcriptional regulator involved in the initiation of neuronal differentiation and mesenchymal to epithelial transition. 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. Together with tcf15, required for the mesenchymal to epithelial transition. Dimers bind DNA on E-box motifs: 5'-CANNTG-3'. Homodimer. Heterodimer; efficient DNA binding requires dimerization with another bHLH protein (By similarity). Interacts with tgfb1i1 (PubMed:16291758). Isoform E12 and isoform E47 are present at all the stages of development and in all regions of the embryo.