AHRR (aryl hydrocarbon receptor repressor) is a transcription factor that functions as a negative regulator of aryl hydrocarbon receptor (AHR) signaling 1. AHRR inhibits AHR transcriptional activity through protein-protein interactions and DNA binding within the AHR complex 2. The gene contains ten exons encoding a 2094-bp mRNA 3. Mechanistically, AHRR modulates cellular responses to xenobiotic exposure and environmental ligands. AHRR silencing restores Th17-cell response to AHR ligands like l-kynurenine in autoimmune hepatitis, suggesting AHRR dysregulation contributes to immune pathology 2. DNA methylation changes at AHRR loci (cg14647125, cg23916896) are associated with smoking exposure and mediate effects on inflammatory bowel disease risk 4. AHRR serves as a robust epigenetic biomarker for cigarette consumption, with methylation at cg05575921 predicting smoking status with high accuracy (AUC=0.8) and showing reversibility after cessation 5. In cancer, elevated AHRR expression in primary high-grade serous ovarian carcinoma correlates with better patient survival and demonstrates anti-tumorigenic function 6. AHRR variants are associated with cleft lip/palate etiology through dioxin-mediated AHR pathway dysregulation and interact with developmental genes controlling palatogenesis 7. These findings establish AHRR as a critical regulatory node integrating environmental exposure responses with immune and developmental processes.