One of the most important clinical applications of HSV-1-tk in CAR-T cells was published by Keu et al. (2017), which demonstrated that tumor-associated CD8+ engineered T lymphocytes expressing both HSV1-tk and interleukin-13 (IL-13) zetakine chimeric antigen receptor (CAR) could be tracked longitudinally by [18F]FHBG imaging in patients with high-grade glioma [96] (Figure 2A). This evidence concerns the gene TKT and central nervous system cancer.