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. Homodimer, and heterodimer with other HLH proteins. Interacts with CLOCK and ARNTL/BMAL1 (By similarity). Interacts with COPS5 and COPS7A. Interacts with IFI204. Interacts with GATA4 and NKX2-5. Interacts with ANKRD2; both proteins cooperate in myoblast differentiation. Nuclear in proliferating cells, translocates to cytosol during cell differentiation. Expressed by myoblasts (at protein level). By a variety of mitogenic agents in serum starved cells. Expressed in a circadian manner in the suprachiasmatic nucleus (SCN) of the brain and heart with peak levels between CT16 and CT20 in SCN and between CT8 and CT12 in the heart. Polyubiquitinated; which is favored by Ifi204 and leads to proteasomal degradation.