CYP2E1 and head and neck squamous cell carcinoma: Genetic polymorphisms in cytochrome P450 (i.e., CYP1A1, CYP1B1, CYP2D6, and CYP2E1) and N-acetyltransferase (NAT1 and NAT2) enzymes involved in the biotransformation of the carcinogenic constituents of tobacco have been shown to be the risk factors involved in HNSCC [20–23].