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). Expressed primarily in cells that have undergone terminal differentiation including notochord, floor plate and cement gland. Localized throughout the animal hemisphere of early cleavage stage embryos and becomes localized above the dorsal blastopore lip during early gastrulation. As gastrulation proceeds, the expression extends along the dorsal midline. In stage 17 embryos, expressed in the notochord and floor plate. At this stage, also detected in the cement gland, where they are maintained in later developmental stages. Additional expression is also detected in the pineal gland, as well as the trigeminal and geniculate ganglia. In stage 27 embryos, expressed in the floor plate and in addition, present in the hypochord and the marginal zone of the ventral neural tube where postmitotic neurons are located.