The second way by which tumor cells can circumvent the cytotoxic action of chemotherapeutic drugs is the increased detoxification by metabolizing enzymes, antioxidation enzymes, etc. In resistant tumor cells, gene overexpression was found in drug metabolizing enzymes such as glutamate–cysteine ligase (GCL) and glutathione S-transferases (GSTs) [14, 15]. This evidence concerns the gene HPGDS and neoplasm.