May act as a transcriptional activator that promotes transcription of muscle-specific target genes and plays a role in muscle differentiation. Efficient DNA binding requires dimerization with another bHLH protein. Expressed in fast and myotomal muscle. Very weak expression in brain, skin and gonads. Most abundant isoform. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay.