Cytotoxic effect of in vivo administration of GCV was examined in mice injected with TK-transduced cells in one flank and with untransduced cells in the other flank. GCV was injected, and the animals were observed for tumor development. All 10 flanks injected with untransduced cells developed tumors (>25 mm), whereas only 1 tumor developed in flanks injected with transduced cells and reached only 5 mm in size.