CYP1A1 and lung carcinoma: Polymorphisms of cytochrome P450, Cyp1A1 and Cyp1A2, and the GST isoenzyme GSTM1 could have implications for polycyclic aromatic hydrocarbon metabolism.48 This may explain the increases in the probability of lung cancer among smokers of more than 20 pack-years, as found in this study.