Abstract: A method is described for the creation of novel isoforms of the enzyme glutathione S-transferase which have enhanced activity in host cells against specific toxic agents. The method includes site directed mutagenesis and selection with the targeted agent in the host cells. The sites of directed mutagenesis is the site of electrophile binding by the native form of the enzyme. This site has proven susceptible to manipulation without loss of enzymatic activity. Various techniques for enhancing the expression, activity, or localization of the expressed enzyme in mammalian cells are described. Genes for the mutant isoforms of the enzyme may be useful in cancer therapeutics to confer upon selected groups of cells heightened resistance to antineoplastic agents.