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. Restricts the CLOCK and ARNTL/BMAL1 localization to the cytoplasm. Plays a role in both the input and output pathways of the circadian clock: in the input component, is involved in modulating the magnitude of photic entrainment and in the output component, contributes to the regulation of a variety of liver clock-controlled genes involved in lipid metabolism. Heterodimer with other HLH proteins. Interacts with NR0B2. Interacts with CLOCK and ARNTL/BMAL1 (By similarity). Interacts with GATA4, IFI204 and NKX2-5. Expressed in a circadian manner in the liver with peak levels seen at CT12 (at protein level). 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. The bHLH domain is essential for its repressor activity towards the CLOCK-ARNTL/BMAL1 heterodimer. Polyubiquitinated; which is favored by Ifi204 and leads to proteasomal degradation. Mice exhibit disrupted locomotor rhythms, reduced locomotor activity, enhanced phase shifts and an increased rate of entrainment when subjected to a large delay of the photoschedule. Show a reduction in lipid storage in the liver and white adipose tissue. Extended N-terminus.