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. May be involved in the regulation or modulation of smooth muscle contraction of the uterus during pregnancy and particularly around the time of delivery. Seems to play a role in the inhibition of myogenesis. Forms inactive heterodimeric complex with TCF3. Highly expressed in the uterus (predominantly in myometrium), ovary, and testis. Expression in the uterus is higher in the diestrus phase than in the estrus phase and reaches a maximum at 7.5 dpc. Expression declines towards the time of delivery and returns to the non-pregnant level 4 days after delivery. Low expression seen in lung, heart, intestine, and spleen. Both the bHLH region and the C-terminal portion are essential for inhibitory function.