The aryl hydrocarbon receptor repressor (AHRR) is predominantly localised in the nucleus of cells (Kanno et al. 2007) and can bind AHR nuclear translocator (ARNT). The resultant AHRR:ARNT complex can compete with AHR:ARNT for binding to the xenobiotic response element (XRE) on target genes such CYP1A1 (Evans et al. 2008, Hahn et al. 2009).