Figure 5. Non-invasive demonstration of transfection of the herpes simplex virus thymidine kinase (HSV-Tk) gene into a murine tumour using PET. The C6tk+ tumour cells were transfected with HSV-Tk and implanted into nude mice. Control tumours had C6 cells only. HSV-Tk causes phosphorylation and intracellular trapping of the radiotracer 9-[(1-[¹⁸F]Fluoro-3-hydroxy-2-propoxy)methyl]guanine ([¹⁸F]FHPG). Note persistence of activity in the C6tk+ tumour indicating gene transfection. Reproduced with permission [91].