Transcriptional repressor. Binds to the canonical and non-canonical E box sequences 5'-CACGTG-3'. Binds DNA as a heterodimer with TCFL4/MLX. Expressed in the ventricular and intermediate zones of the developing spinal cord of 12.5 dpc embryos. In later embryos expressed in a variety of tissues. Phosphorylation at Ser-566 by AMPK inactivates the DNA-binding activity.