As mentioned in Section 2, antitumor drug-metabolizing CYPs may be aberrantly expressed in tumor cells, because of their involvement in tumor physiology and pathology, such as the overexpression of both CYP1B1 in breast cancer cells and CYP2A6 in liver and lung cancers [71,72,73,74]; while, the expression of some CYPs involved in the development of liver ischemia, reperfusion, and sepsis are decreased [75]. This evidence concerns the gene CYP2A6 and neoplasm.