CYP1A1 is a phase I xenobiotic-metabolizing enzyme that bioactivates environmental procarcinogens, particularly polycyclic aromatic hydrocarbons (PAHs), into reactive intermediates that can initiate carcinogenesis. Its expression is primarily regulated at the transcriptional level by the aryl hydrocarbon receptor (AHR), which upon ligand binding induces CYP1A1 via xenobiotic response elements. Genetic polymorphisms in the human CYP1A1 gene have been linked to increased susceptibility to lung and other cancers.