Transcriptional repressor. MXI1 binds with MAX to form a sequence-specific DNA-binding protein complex which recognizes the core sequence 5'-CAC[GA]TG-3'. MXI1 thus antagonizes MYC transcriptional activity by competing for MAX. Isoform Short, which lacks a segment, has a much stronger suppressive potential and associates with a SIN3 homologous protein. Efficient DNA binding requires dimerization with another bHLH protein. Binds DNA as a heterodimer with MAX. Interacts with SMC3. Interacts with RNF17. Primarily expressed in differentiating cells. Also expressed in the proliferating cells of the developing CNS and the epidermis. In the spinal cord at embryonic day 10.5, expressed in the ventricular zone of the neural tube. Expressed at highest levels in cells accumulating in the intermediate zone. At 11.5 and 12.5 dpc, highly expressed in the intermediate zone and at reduced levels in the ventricular zone that mostly persists in the dorsal part of the neural tube. At 14.5 dpc, expressed throughout the spinal cord. In the developing epidermis at 14.5 dpc, found in the dorsal lateral epidermis. At 17 dpc, expression is restricted not only to the differentiating cells of the epidermis but also to the proliferating cell compartment and expression extends into the first differentiating cell layers, but decreases in the uppermost layers of the epidermis.