Essential for cardiac morphogenesis and for the development of branchial arches. Binds DNA on E-box consensus sequence 5'-CANNTG-3' (By similarity). Efficient DNA binding requires dimerization with another bHLH protein. Heart, liver and spleen. At stage 15, faint expression in the antero-ventral mesoderm. By stage 20, after neural tube closure, expressed in the ventro-lateral mesoderm of the mid-embryo with punctate expression in the anterior clusters of subepidermal cells, which may represent migrating neural crest cells. By stage 24 (early tailbud stage), the expression pattern expands to include lateral plate mesoderm in the mid-embryo, symmetrically along the left-right axis, and the heart anlage in the ventral midline. By stages 28-30 (late tailbud stages), the region of expression in the lateral plate mesoderm has expanded into a symmetrical arc of ventral and lateral expression, which fuses in the midline where the straight heart tube has formed. At the same stages, branchial arches present high levels of expression. By stage 33 (beginning of heart looping), expressed in the heart, lateral mesoderm and branchial arches. The expression persists in the looped heart and the branchial arches throughout stage 37. Most abundant in the heart and outflow tract at stage 47 (tadpole stage).