The HSV-1-tk system has been further elaborated in the past 15 years with regard to (i) the assessment of anti-cancer therapy paradigms [89,90,91,92], employing tumor-specific promoters [93,94], cancer-directed T-cells [95,96], transduced mesenchymal stem cells [97,98,99,100] or bone marrow-derived tumor-infiltrating progenitor cells; (ii) the visualization of tumor burden [101] and the primary anti-tumor immune response [102]. The gene discussed is TKT; the disease is cancer.