Experimental outline. (A) Schematic representation of the retroviral suicide gene vector MO3TIN and the EL-4 tumor model used in this study. The experiments performed to address the potential mechanisms of escape from ganciclovir (GCV) suicide treatment are summarized in the box. (B) Determination of the in vivo doubling time of tumorigenic EL-4 cells in C57Bl/6 mice. A regression line was extrapolated after plotting on the y-axis the values of the logarithms₁₀ for administered EL-4 cells with numbers of 1 × 10⁴ (n = 7); 5 × 10⁴ (n = 5); 1 × 10⁵ (n = 12); 5 × 10⁵ (n = 5); and 1 × 10⁶(n = 12) versus days until 50% tumor incidence (according to the method of Skipper).²⁴