Some antiviral drugs, such as acyclovir (ATC: J05AB01) and ganciclovir (ATC: J05AB06) as well as other nucleoside analogs make use of the substrate specificity of viral thymidine kinase, as opposed to human thymidine kinases. These drugs act as pro-drugs, which in themselves are not toxic, but are converted to toxic drugs by phosphorylation by viral thymidine kinase. Cells infected with the virus therefore produce highly toxic triphosphates that lead to cell death. Human thymidine kinase, in contrast, with its more narrow specificity, is unable to phosphorylate and activate the prodrug. In this way, only cells infected by the virus are susceptible to the drug. Such drugs are effective only against viruses from the herpes group with their specific thymidine kinase. In patients treated with this type of drugs, the development of antiviral drug resistance is frequently observed. Sequencing the thymidine kinase gene in Herpes simplex virus and Varicella zoster virus shows the rapid genetic variability and may facilitate the diagnosis of antiviral drug resistance.