Early transcription factor that plays a key role in somitogenesis, paraxial mesoderm development and regulation of stem cell pluripotency (PubMed:7597044, PubMed:8955271, PubMed:23395635, PubMed:32669716, PubMed:24038871). Essential for the mesenchymal to epithelial transition associated with somite formation (PubMed:7597044, PubMed:8955271, PubMed:24038871). Required for somite morphogenesis, thereby regulating patterning of the axial skeleton and skeletal muscles (PubMed:8955271). Required for proper localization of somite epithelium markers during the mesenchymal to epithelial transition (PubMed:24038871). Also plays a key role in regulation of stem cell pluripotency (PubMed:23395635, PubMed:32669716). Promotes pluripotency exit of embryonic stem cells (ESCs) by priming ESCs for differentiation (PubMed:23395635). Acts as a key regulator of self-renewal of hematopoietic stem cells (HSCs) by mediating HSCs quiescence and long-term self-renewal (PubMed:32669716). Together with MEOX2, regulates transcription in heart endothelial cells to regulate fatty acid transport across heart endothelial cells (PubMed:25561514). Acts by forming a heterodimer with another helix-loop-helix (bHLH) protein, such as TCF3/E12, that binds DNA on E-box motifs (5'-CANNTG-3') and activates transcription of target genes (PubMed:15226298, PubMed:23395635). Heterodimer; efficient DNA binding requires dimerization with another bHLH protein, such as TCF3/E12 (PubMed:15226298, PubMed:23395635). Interacts with MEOX2 (PubMed:25561514). Expressed in heart and skeletal muscle (PubMed:8041747, PubMed:7597044). Specifically expressed in a subpopulation of embryonic stem cells (ESCs), that are still undifferentiated but primed for ifferentiation (PubMed:23395635). Expressed in hematopoietic stem cells (HSCs) (PubMed:32669716). Expressed in the embryonic day 4.5 embryo (PubMed:23395635). Expressed in paraxial mesoderm and developing somites (PubMed:7729571). At 7.5 dpc, low expression is detected in a subdomain of the primitive mesoderm (PubMed:8041747). At 8.5 dpc, expressed at high levels throughout the uncompartmentalized epithelial somites and in the rostral paraxial mesoderm (PubMed:8041747). By 9.5 dpc, expression is confined to the somite and is most prominent in the myotome and dermatome (PubMed:8041747). At 10 dpc, expression in somite declines in the myotome but persists at high level in the dermatome (PubMed:8041747). At 10.5 dpc, expression is seen only in the somites in the caudal portion of the embryo (PubMed:8041747). Lethality within an hour of birth, possibly caused by rib defects that lead to respiratory distress (PubMed:8955271). At birth, neonates display obvious caudal agenesis with tails that are shortened and curled (PubMed:8955271). They also lack the normal cervical flexure of the vertebral column, and their hindlimbs are curved towards the midline (PubMed:8955271). Neonates also display low-set ears, a thickened neck and loose skin (PubMed:8955271). Defects are caused by impaired formation of somites: cells from the paraxial mesoderm are unable to form epithelia, preventing formation of somites (PubMed:8955271). In the absence of normal somites, the axial skeleton and skeletal muscle form but are improperly patterned (PubMed:8955271). Hematopoietic stem cells (HSCs) show a specific loss of quiescent stem cells (PubMed:32669716).