For many years, antibodies have been examined as means to deliver cytotoxic proteins to kill target cells (immunotoxins). More recently, there have been studies on enzymes that convert prodrugs to active drugs to kill target cells. The advances in gene therapy strategies now allow one to deliver the gene for the protein or enzyme as an alternative. This technique, although in its infancy, promises to overcome some of the problems associated with antibody-mediated delivery. Thymidine kinase and cytosine deaminase are some of the enzymes currently being exploited in this way, but more are on the horizon. However, more research is still needed to enable full exploitation of the transcriptional differences between tumour and normal cells so that more existing cancers can be treated in this way.