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). . This is the function of TCF23 (transcription factor 23, Ensembl gene identifier ENSG00000163792).