Important for striated muscle differentiation and cardiac morphogenesis. Is also required for cardiac conduction system development, plays a regulatory function in heart rate dynamics mediated, at least in part, through cAMP-binding. Expressed in the heart and, slightly, in skeletal muscle. Detectable at 4 hours post-fertilization (hpf), expression levels peaked at 24 hpf and gradually decreased afterwards. Expression become detectable when skeletal muscle differentiation commences at 14-16 somites with an anteropsterior gradient. Myocardial expression starts at 22 hpf in the forming heart tube and, at 24 hpf, is confined to differentiated cardiac muscles cells. At 24 hpf, is also expressed in the entire myotome. At 48 and 72 hpf, expression in the trunk muscle cells disappeares, but is present in muscles cells of the pectoral fin bud and facial muscles. Morpholino knockdown of the protein affects skeletal muscle development, resulting in abnormal facial and trunk muscle formation with defective aligment of muscle fibers and establishment of myotendinous junctions. A severe defect is observed in craniofacial muscle development, where many muscles are malformed or reduced in size. In the heart, pericardial edema is prevalent in the morphants with defective looping and misshaped chambers. Belongs to the popeye family.