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. Inhibits the activity of both neurogenic (neurod1/neuroD) and myogenic (myod1/myoD) bHLH factors. May play a role in the regulation of the circadian clock (By similarity). Heterodimer with other HLH proteins.