The CYP1A1 gene is transcribed to yield mRNA and the mRNA is translated to yield protein (van der Meer et al. 2010, Rakhshandehroo et al. 2009). The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that controls the expression of a diverse set of genes. AHR acts as a sensor of polyaromatic chemicals, which upon activation, translocates to the nucleus and binds with the aryl hydrocarbon receptor nuclear translocator (ARNT) to transactivate target genes via binding to xenobiotic response elements (XREs) in their promoter regions. AHR targets genes of Phase I and Phase II metabolism, such as cytochrome P450 1A1 (CYP1A1) (Beischlag et al. 2008). AHR is regulated by the aryl hydrocarbon receptor repressor (AHRR, aka BHLHE77, KIAA1234) that inhibits both xenobiotic-induced and constitutively active AHR transcriptional activity in many species (Hahn et al. 2009, Haarmann-Stemmann & Abel 2006).