After HSV-TK gene was transfected into tumor cells, its encoding product (thymidine kinase) can phosphorylate non-toxic GCV to cytotoxic GCV triphosphate (GCV-TP), which is involved in the DNA replication and leads to an advanced replicate termination with the following apoptosis of cancer cells (Figure 3; Chen et al., 2014; Zeng et al., 2014). Here, TKT is linked to cancer.