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. Heterodimer with other HLH proteins. Expressed in a circadian manner in the suprachiasmatic nucleus (SCN) of the brain with peak levels between CT16 and CT20. Mice do not exhibit a noticeable circadian phenotype.