Studies have shown that patients carrying a mutation of the CYP1A1/2, CYP2D6, and CYP2E1 have an increased risk of head and neck squamous cell carcinoma, esophageal carcinoma, and lung carcinoma, arising from a deficiency in metabolizing carcinogens to their inactive derivatives (55, 56); alcohol and nicotine are common carcinogens. Here, CYP2E1 is linked to head and neck squamous cell carcinoma.