Transcription factor that plays a role in the development of the hypothalamo-pituitary axis. Specifically recognizes the xenobiotic response element (XRE) (By similarity). Isoform 1 acts as a transcriptional activator (PubMed:11072074). Isoform 3 acts as a transcriptional repressor (PubMed:10960127, PubMed:11072074). Efficient DNA binding requires dimerization with another bHLH protein. Heterodimer with the aryl hydrocarbon receptor (AHR), SIM1 or HIF2A/EPAS-1. Isoform 1 and isoform 2 are most highly expressed in the brain, eye and skeletal muscle and to a lower degree in liver, heart, kidney and swim bladder. Isoform 3 is most highly expressed in the eye, forebrain, midbrain, hindbrain, skeletal muscle, gills and brain but is barely detectable in liver, heart, kidney and swim bladder. Before the pharyngula period isoform 3 is expressed throughout the entire embryo and during this period extensively in the brain and eye.