Isoform 1 specifically recognizes the xenobiotic response element (XRE), functions positively in AHR-mediated signaling and activates transcription. Isoform 2 has reduced affinity for the XRE and functions negatively in AHR-mediated signaling by disrupting the binding of dimers formed by isoform 1 and AHR to the XRE. Efficient DNA-binding requires dimerization with another bHLH protein. Heterodimer with the aryl hydrocarbon receptor (AHR). In day 23 embryos, expressed at highest levels in brain, spinal cord and epithelial cells of developing gill. Expressed at higher levels throughout all tissues in day 27 and 35 sac fry, predominantly in brain and spinal cord. In the adult, isoform 1 is widely expressed (at protein level) and is detected in liver, gill, gonad, brain, kidney, heart, spleen and muscle. Isoform 2 is expressed (at protein level) only in gills with highest expression in the epithelial cells sourrounding the gill lamellae. Isoform 1 is expressed both in juvenile and adult with highest levels in day 42 sac fry. Isoform 2 is only expressed in adult (at protein level). The C-terminal region of isoform 1 contains a transactivation domain. The Pro/Ser/Thr-rich region in the C-terminus of isoform 2 inhibits DNA-binding of ARNT-AHR dimers. Expressed at much lower levels than isoform 1.