There are different classes of drugs directed against thymidine metabolism and thereby involving thymidine kinase that are used to control cancer associated cell division. Chain terminators are thymidine analogues that are included in the growing DNA chain, but modified so that the chain cannot be further elongated. As analogs of thymidine, this type of drugs are readily phosphorylated to 5'-monophosphates. The monophosphate is further phosphorylated to the corresponding triphosphate and incorporated in the growing DNA chain. The analog has been modified so that it does not have the hydroxyl group in the 3'-position that is required for continued chain growth. In zidovudine (AZT; ATC:J05AF01) the 3'-hydroxyl group has been replaced by an azido group, in stavudine (ATC: J05AF04) it has been removed without replacement. AZT is used as substrate in one of the methods for determination of thymidine kinase in serum. This implies that AZT interferes with this method and may be a limitation: AZT is a standard component of HAART therapy in HIV infection. One common consequence of AIDS is lymphoma and the most important diagnostic application of thymidine kinase determination is for monitoring of lymphoma.