Gene delivery, the insertion of genes (e.g. via retroviral vectors) into selected cells in the body in order to: Cause those cells to produce specific therapeutic agents;Cause those cells to become (more) susceptible to a conventional therapeutic agent that previously was ineffective against that particular condition/disease;Cause those cells to become less susceptible to a conventional therapeutic agent;Counter the effects of abnormal (damaged) tumour suppressor genes via insertion of normal tumour suppressor genes;Cause expression of ribozymes that cleave oncogenes (cancer-causing genes);Introduce other therapeutics into cells.