Transcription factor, which play a role in limb development. Is an essential player in the regulatory network governing transcription of genes implicated in limb morphogenesis. Heterodimer. Efficient DNA binding requires dimerization with another bHLH protein. Interacts with TCF3, TCF4, and TCF12. At 10.5 dpc, expressed in forelimb and hindlimb buds, in the distal mesenchyme below the apical ectodermal ridge. At 11.5 dpc, expression is restricted to the subridge mesenchymal layer, as well as in the dorsal and ventral regions of the developing limbs.