Transcription repressor that blocks myogenesis and activation of E-box dependent muscle genes. Efficient DNA binding requires dimerization with another bHLH protein. Binds DNA as a homodimer or a heterodimer. Forms a heterodimer with TCF3. Expression in embryos is largely restricted to embryonic skeletal muscle lineage. First detected at 9.5 dpc in cells within the first and second branchial arches. At 10.5 dpc, expression was also observed in the myotomal compartment of rostral somites and by 11.5 dpc in myotomes along the whole anterio-posterior axis, as well as in cells within the developing forelimbs and hindlimbs. At 12.5-14.5 dpc, expression was confined to the skeletal muscle lineage (head, neck, trunk, limbs and diaphragm but not cardiac and smooth muscle). At 15 dpc, a peak expression was seen in neonatal limbs.