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. Influences cell fate decisions in the embryo by sequestering and blocking the activity of the bHLH transcription factors that control these decisions. Inhibits the binding of myogenic bHLH-containing complexes to E-box DNA, thereby preventing activation of muscle-specific target genes. Also inhibits the activity of neurogenic factor neurod1/neuroD. Plays a role in cell cycle progression and survival of neural crest progenitors; binding to either hes4-B/hairy2b or stat3 blocks the formation of transcription factor complexes and the repressor function of hes4-B/hairy2B, to allow neural crest progenitors to differentiate. May play a role in the regulation of the circadian rhythm (By similarity). Homodimer (By similarity). Heterodimer with other HLH proteins. Interacts (via HLH domain) with the bHLH protein hes4/hairy2 (via Orange domain). Interacts with stat3 (By similarity).