Suicide gene therapy examples include pro-apoptotic gene therapy, gene-directed prodrug therapy, imaging/targeted radiotherapy and immunotherapy. Oncolytic viral therapy places viral genes, which are critical for replication, under the control of the hTERT gene promoter. This results in virus vectors that are able to replicate only in cancer cells, and can then spread to adjacent cancer cells on cell lysis. To date, most studies have taken advantage of the versatile suicide gene therapy approach to use both the hTR and hTERT promoters to regulate a wide variety of cell killing agents.