Treatment of CT26 tumors by EGT with HS-tk-GCV approach induces strong growth inhibition. Mice, bearing CT26 tumors, were treated with intratumoral injection of K-PBS (A and B) or 50 μg of pCEP4/TK (C and D) with in vivo EP. All mice received 2 units of twice the amount of EGT or mock procedures on days 0, 2, 8, and 10 and i.p. injection of GCV (3 mg/100 g b.w. × 2/day) from day 2 to day 16. Large tumors (A and B) developed in the control mice [{(P−E+) × 2} × 2] (arrows). No tumor (C) or a very small tumor (D) was seen at the inoculation site in the EGT-treated mice [{(P+E+) × 2} × 2] (arrows). Photos were obtained on day 16.