Essential for myocardial and pectoral fin differentiation, patterning and morphogenesis. Efficient DNA binding requires dimerization with another bHLH protein. Expression first detected at 8 hours post fertilization (hpf) in a circumferential pattern in the lateral mesoderm. At 12 hpf, still expressed in the anterior and posterior lateral plate mesoderm which is partly detached from the yolk at this stage. By 18-20 hpf, expressed in the cardiac ring and in a layer of lateral mesoderm surrounding the cardiac region, as well as in the posterior lateral mesoderm. At 24 hpf, expressed throughout the nascent cardiac tube and in bilaterally symmetric clusters of ventrolateral cells that condense to form branchial arch mesoderm which gives rise to jaw and pharyngeal bones. Expression continues bilaterally in the posterior lateral mesoderm. At 36-48 hpf, expressed in defined branchial arch mesoderm populations. At 48 hpf, strong expression throughout the heart tube.