The herpesvirus thymidine kinase gene has also been used as a “suicide gene” as a safety system in gene therapy experiments, allowing cells expressing the gene to be killed using ganciclovir. This is desirable in case the recombinant gene causes a mutation leading to uncontrolled cell growth (insertional mutagenesis). The cytotoxic products produced by these modified cells may diffuse to neighboring cells, rendering them similarly susceptible to ganciclovir, a phenomenon known as the "bystander effect." This approach has been used to treat cancer in animal models, and is advantageous in that the tumor may be killed with as few as 10% of malignant cells expressing the gene. A similar system has been tried using tomato thymidine kinase and AZT. In addition, thymidine kinase gene is used as a suicide gene to tackle dangerous graft-versus-host disease in hematopoietic stem cell transplant therapy named Zalmoxis that was conditionally approved in Europe in 2016