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 (neurog1/neurogenin, neurod1/neuroD) and myogenic (myod1/myoD) bHLH factors. Heterodimer with other HLH proteins. During embryonic development, expressed in a number of neural tissues, including Rohon-Beard neurons, olfactory placode, eye primordia, and the trigeminal ganglia. Also expressed in other organs including the pronephros and liver primordium. Pronephric development begins by stage 25 and increases during tailbud stages. Expressed in both the tubules and the duct. As embryogenesis progresses, expressed in the migrating melanocytes and lateral line structures. By bmp4. Inhibited by Notch-signaling.