A suicide gene, in genetics, will cause a cell to kill itself through apoptosis. Activation of these genes can be due to many processes, but the main cellular "switch" to induce apoptosis is the p53 protein. Stimulation or introduction (through gene therapy) of suicide genes is a potential way of treating cancer or other proliferative diseases. Suicide genes form the basis of a strategy for making cancer cells more vulnerable, more sensitive to chemotherapy. The approach has been to attach parts of genes expressed in cancer cells to other genes for enzymes not found in mammals that can convert a harmless substance into one that is toxic to the tumor. Most suicide genes mediate this sensitivity by coding for viral or bacterial enzymes that convert an inactive drug into toxic antimetabolites that inhibit the synthesis of nucleic acid. Suicide genes must be introduced into the cells in ways that ensure their uptake and expression by as many cancer cells as possible, while limiting their expression by normal cells. Suicide gene therapy for cancer requires the vector to have the capacity to discriminate between target and non target cells, between the cancer cells and normal cells.