The original idea behind gene therapy was curing hereditary defects by replacing defective genes. However, there is no inherent reason why gene therapy must only be “defensive” and suppress defects. We can go on the offensive and provide genes whose products may cure a disease even though the genes we use were not responsible for the problem in the first place. The best examples of such aggressive gene therapy are not in curing hereditary defects but in the treatment of cancer. Here the objective is to kill cancer cells. In fact, the majority of gene therapy trials are now directed against cancer.