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. Efficient DNA binding requires dimerization with another bHLH protein. Binds DNA as a heterodimer with MAX. Barely detectable during early stages of active growth and differentiation, exhibits dramatic increase between 30 and 48 hours of development. This late embryonic stage marks a period of continued maturation and moderate growth of most organs, but is also characterized by initiation of cellular growth arrest and terminal differentiation in many cell types.