TKT and neoplasm: We hypothesized that green fluorescent protein (GFP)- or red fluorescent protein (RFP)-only positive cells are equally important as the GFP/RFP dual-positive cells in assessing sp-RRV dissemination, as GSCs transduced with only one therapeutic gene (TK or CD) show considerable antitumor abilities owing to the enhanced bystander effects caused by phosphorylated GCV or 5-FU [8,9,14,23,41,42,43]; in such cases, these cytotoxic molecules can be delivered to the same tumor cells or nearby non-transduced tumor and tumor-promoting stromal cells.