The aim of this study was to assess the efficacy hMSC for targeting microscopic tumors and suicide gene or cytokine gene the rapy.Immunodeficient mice were transplanted subcutaneously (s.c.) with human colon cancer cells ofHT-29 Inv2 or CCS line and 3-4 days later intravenously with "tracer" hMSCs expressing herpes simplex virus type 1 thymidine kinase (HSVTK) and eGFP reporter genes.Subsequently,these s.c.tumors were examined for the specificity and magnitude of HSVTK+,eGFP+ stem cell engraftment and proliferation into tumor stroma by in vivo p ositron emission tomography (PET) with 18F-FHBG.In vivo PET images of tumors growing for 4 weeksdemons trated the presence of a significant portion of HSVTK+ tumor stroma with an average of 5.78 ± 4.79 %ID/g 18F-FHBG accumulation.In vivo imaging results were validated by in situ correlative histochemical,immuno-fluorescent,and-cytometric analyses,which revealed eGFP expression in vWF+ and CD31 + endothelial cells of capillaries and larger blood vessels,in germinal laver of dermis and hair follicles proximal to the s.c.tumor site.