TKT and cancer: Consistent with the high cytotoxicity of Ad-ERE73 (1 + 2)-ARF (−13)-TK and Ad-ERE73 (3 + 4)-ARF (−13)-TK in cancer cell lines, the HSV-TK gene is expressed in cancer cells at levels 10- to 100-fold greater than that in normal HFFs, even at an order of magnitude lower viral MOI (Figure 3B).