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. Involved in myogenesis by inhibiting skeletal muscle and cardiac myocyte differentiation and promoting muscle precursor cells proliferation. Inhibits the binding of E2A-containing protein complexes to muscle creatine kinase E-box enhancer. Regulates the circadian clock by repressing the transcriptional activator activity of the CLOCK-ARNTL/BMAL1 heterodimer (By similarity). Homodimer, and heterodimer with other HLH proteins. Interacts with COPS5 and COPS7A. Interacts with IFI204. Interacts with GATA4 and NKX2-5. Interacts with ANKRD2; both proteins cooperate in myoblast differentiation. Interacts with CLOCK and ARNTL/BMAL1 (By similarity). Phosphorylated in vitro by CDC2 and PKC.