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. Heterodimer with other HLH proteins. In the embryo, expressed in a range of tissues, with primary expression in the developing pronephros; expressed in the pronephric anlage, and by the swimming tadpole stages expressed robustly in the pronephric tubules and weakly in the pronephric duct. Expressed in the secondary heart field. In the developing nervous system, expressed in the neural crest and in the neural folds during neurula stages, and at stage 20 in the neural tube, ventral mesoderm and mid-hindbrain boundary. By early tailbud stages, expressed in the neural tube, somites and branchial arches. In tadpoles (stage 37/38), expressed in the heart, eye, otic vesicle, somites and branchial arches. Also expressed in migrating muscle cells. Expressed at a low level in limbs, with expression decreasing as limbs develop, but expressed at a high level in blastemas (regenerated limbs), where expression is localized primarily to the blastemal epidermis. Widely expressed in adults with highest expression in the spleen, skin, intestine and brain, and at a much lower level in testis and heart. Expressed both maternally and zygotically. Maternal expression declines during cleavage and blastula stages with zygotic expression increasing from gastrulation. From neurula stage onwards, expression levels are relatively constant. May be induced by Notch signaling.