Transcriptional regulator (lacking a basic DNA binding domain) which negatively regulates the basic helix-loop-helix (bHLH) transcription factors by forming heterodimers and inhibiting their DNA binding and transcriptional activity. Implicated in regulating a variety of cellular processes, including cellular growth, senescence, differentiation, apoptosis, angiogenesis, and neoplastic transformation. Inhibits skeletal muscle and cardiac myocyte differentiation. Regulates the circadian clock by repressing the transcriptional activator activity of the CLOCK-ARNTL/BMAL1 heterodimer (By similarity). Heterodimer with other HLH proteins. Interacts with COPS5, IFI204, GATA4, NKX2-5, CLOCK and ARNTL/BMAL1 (By similarity). Isoform Short can form homodimers. Additional isoforms seem to exist. Phosphorylated in vitro by PKA and PKC.