CYP1A1 and obesity due to melanocortin 4 receptor deficiency: The AHR/ARNT heterodimer regulates the transcription of genes in the cytochrome P450 Cyp1 family, some phase II detoxification genes, and thousands of other genes (Trask et al. 2009), including the gene expression of other nuclear receptors relevant to obesity [e.g., Ppara (peroxisome proliferator–activated receptor-α)] (Wang et al. 2011).