CYP1A1 and lip and oral cavity carcinoma: CYP1A1 rs4646903 and rs1048943 polymorphisms contribute to increased enzyme activity of CYP1A1 and are crucial to the activation of PAHs.6,39 The null genotype of GSTM1 polymorphism could result in the inactivation of GSTM1 enzyme and thus decrease the capacity of detoxifying carcinogens.71 So far, several epidemiological studies have evaluated the association of CYP1A1 and GSTM1 polymorphisms with oral cancer susceptibility.