ID (inhibitor of DNA binding) HLH proteins lack a basic DNA-binding domain but are able to form heterodimers with other HLH proteins, thereby inhibiting DNA binding. 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 (By similarity).