Inhibits E-box-mediated binding and transactivation of bHLH factors. Inhibitory effect is similar to that of ID proteins. Inhibits the formation of TCF3 and MYOD1 homodimers and heterodimers. Lacks DNA binding activity. Seems to play a role in the inhibition of myogenesis (By similarity). Forms inactive heterodimeric complexes with TCF3. Both the bHLH region and the C-terminal portion are essential for inhibitory function.