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. Heterodimer with other HLH proteins. Interacts with CLOCK and ARNTL/BMAL1 (By similarity). Interacts with COPS5, IFI204, GATA4 and NKX2-5. Additional isoforms seem to exist. Expressed in a circadian manner in the suprachiasmatic nucleus (SCN) of the brain and heart with peak levels seen between CT16 and CT20 in the SCN and between CT8 and CT12 in the heart. Polyubiquitinated; which is favored by Ifi204 and leads to proteasomal degradation.