Acts as a transcriptional regulator. Inhibits myogenesis by sequestrating E proteins, inhibiting trans-activation by MEF2, and inhibiting DNA-binding by MYOD1 through physical interaction. This interaction probably involves the basic domains of both proteins. Also represses expression of pro-inflammatory cytokines such as TNFA and IL1B. Regulates cranial suture patterning and fusion. Activates transcription as a heterodimer with E proteins. Regulates gene expression differentially, depending on dimer composition. Homodimers induce expression of FGFR2 and POSTN while heterodimers repress FGFR2 and POSTN expression and induce THBS1 expression. Heterodimerization is also required for osteoblast differentiation. Represses the activity of the circadian transcriptional activator: NPAS2-ARNTL/BMAL1 heterodimer (By similarity). Efficient DNA binding requires dimerization with another bHLH protein. Homodimer or heterodimer with E proteins such as TCF3. ID1 binds preferentially to TCF3 but does not interact efficiently with TWIST1 so ID1 levels control the amount of TCF3 available to dimerize with TWIST1 and thus determine the type of dimer formed (By similarity). Subset of mesodermal cells. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry.