expression of the cloned gene. (A, B) HEK293T cells were transfected with Gag/Pol, VSVG, and tdTomato plasmids. The expression of the tdTomato gene was assessed using a fluorescence microscope. Fluorescent images were superimposed on a bright-field image for the differentiation of positively transduced cells. Transfection efficiency was determined based on the expression of tdTomato after 24 hours. Non-transfected HEK293T cells were used as controls (blue histogram). (C, D) The murine leukemia cell line C1498 was transduced with fresh virus. Four days later, transgene expression was assessed by fluorescence microscopy (C) and flow cytometry (D). Non-transduced C1498 cells were used as controls (blue histogram). Scale bars represent 30 μm.