TCF3 contains 1 basic helix-loop-helix (bHLH) domain. 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. Dimers bind DNA on E-box motifs: 5'-CANNTG-3'. TCF3 binds to the kappa-E2 site in the kappa immunoglobulin gene enhancer.