The HSVtk expression results in the phosphorylation of drug nucleoside analogues; in this case the drug ganciclovir, an antiviral medication used to treat and prevent cytomegaloviruses, converts it into the nucleoside analogue triphosphates. Once granciclovir is phosphorylated through HSV-tk it is then incorporating DNA strands when the cancer cells are multiplying.[1] The nucleotide from the ganciclovir is what inhibits the DNA polymerization and the replication process. This causes the cell to die via apoptosis.[2]