Genetic prodrug activation therapy shows promise as a therapeutic option for the treatment of cancer as well as a variety of other diseases. It involves the insertion of a gene encoding a drug-metabolizing enzyme into cells and the systemic administration of a prodrug. The prodrug is converted to a cytotoxic agent by the action of the expressed enzyme. To ensure that the enzyme is only expressed in the targeted subset of cells, the transcriptional apparatus of a gene that is unique to this subset is used to regulate the gene encoding the drug-metabolizing enzyme. As with all types of gene therapy, one of the major obstacles to successful clinical treatment is the development of safe and effective gene delivery systems.