Essential for cardiac morphogenesis, particularly for the formation of the right ventricle and of the aortic arch arteries. Required for vascular development and regulation of angiogenesis, possibly through a VEGF signaling pathway. Also plays an important role in limb development, particularly in the establishment of anterior-posterior polarization, acting as an upstream regulator of sonic hedgehog (SHH) induction in the limb bud. Is involved in the development of branchial arches, which give rise to unique structures in the head and neck. Binds DNA on E-box consensus sequence 5'-CANNTG-3' (By similarity). Efficient DNA binding requires dimerization with another bHLH protein. Forms homodimers and heterodimers with TCF3 gene products E12 and E47, HAND1 and HEY1, HEY2 and HEYL (hairy-related transcription factors) (By similarity). Heart. Expressed in the fetal heart.