Figure 4. Comparative metabolism of the thymidine and ganciclovir by Herpes simplex virus thymidine kinase and cellular kinases. A. Thymidine metabolism. Typically, thymidine kinase enzymes can phosphorylate the thymidine to thymidine triphosphate, for further integration into DNA. B. Metabolism of ganciclovir. The HSV thymidine kinase, unlike to human thymidine kinase, is able to phosphorylate ganciclovir to convert to GCV-P, once this happened, the cellular kinases can phosphorylate it for later integration into the DNA, which leads to the arrest of its synthesis and therefore, the cell death.