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 (By similarity). Heterodimer with other HLH proteins. Interacts with COPS5, IFI204, GATA4 and NKX2-5 (By similarity). Interacts with CLOCK and ARNTL/BMAL1. Additional isoforms seem to exist. Expression correlates with proliferation in some types of cells.