TKT and neoplasm: In our previous preclinical studies using the “suicide gene” herpes simplex virus type 1 thymidine kinase (HSV-TK) [9–11] or the sodium iodide symporter (NIS) [12–14] as therapy genes, we demonstrated the active homing of MSCs to the tumor stroma resulting in a significant reduction in tumor growth and prolonged animal survival after the application of ganciclovir or 131I, respectively.