Cardiac- and muscle-specific transcription factor. May act to regulate the expression of genes involved in the development of myotubes. Plays a critical role in ventricular cardiomyocyte expansion and regulates postnatal skeletal muscle growth and regeneration. Involved in the organized assembly of thick and thin filaments of myofibril sarcomeres (By similarity). Interacts (via PXLXP motif) with the muscle-restricted histone methyltransferase SMYD1 (via MYND-type zinc finger). The proline-rich muscle-specific exon 3 contains eighteen approximate 23-AA repeats.