In vitro and in vivo preclinical studies and phase I/II clinical trials have demonstrated that the retroviral-mediated transfer of the suicide gene HSV-thymidine kinase into donor T-cells prior to infusion (ie, a 2-week ex vivo process including activation, retroviral transduction and selection of transduced cells), at the time of T-cell-depleted hematopoietic stem cell transplantation (HSCT) or as donor lymphocyte infusion after relapse, allows for the efficient control of donor T-cell alloreactivity. These donor suicide gene-modified T-cells (SGMTCs) can provide beneficial anti-leukemic, antiviral and immune reconstitution-facilitating effects to the recipient of an allogeneic HSCT. However, if the infused SGMTCs lead to GvHD, a severe complication of HSCT, these cells can be specifically depleted in vivo by the administration of the prodrug ganciclovir (GCV), without any associated immunosuppression. Limitations to this approach include a gene transfer-induced decrease in alloreactivity and antiviral reactivity, the immunogenicity of SGMTCs, and the development of GCV-resistant SGMTCs. However, major improvements that can prevent these limitations, such as introducing CD3/CD28 costimulation and immunomagnetic selection, have been applied to this approach, but further improvements are still required. The efficacy of suicide gene therapy as a safety control system allows the development of this strategy for gene therapy or immunotherapy approaches.