Component of a transcriptional repressor complex together with MAX (By similarity). In complex with MAX binds to the core DNA sequence 5'-CAC[GA]TG-3' (By similarity). Antagonizes MYC transcriptional activity by competing with MYC for MAX binding (By similarity). Binds to the TERT promoter and represses telomerase expression, possibly by interfering with MYC binding (By similarity). Heterodimer with MAX; the interaction is required for DNA-binding (By similarity). DNA binding requires dimerization with another bHLH protein; does not form homodimers, and does not bind to DNA in the absence of MAX in vitro (By similarity). Interacts with RNF17 (PubMed:10597267). Expressed during neural and epidermal differentiation. Primarily expressed in growth-arrested differentiating cells. In the spinal cord at embryonic day 10.5, a strong expressesion seen in the differentiating cells of the intermediate zone at the ventral part of the neural tube and weakly in the ventricular 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 17.5 dpc expressed in the cell cycle arrested, differentiating cells of the suprabasal malphigian layer. Ubiquitinated by BIRC2/c-IAP1, leading to its subsequent degradation by the proteasome.