Thymidine kinase is an enzyme, a phosphotransferase: 2'-deoxythymidine kinase, ATP-thymidine 5'-phosphotransferase, EC 2.7.1.21. It can be found in most living cells. It is present in two forms in mammalian cells, TK1 and TK2. Certain viruses also have genetic information for expression of viral thymidine kinases. Thymidine kinase catalyses the reaction: Thd + ATP → TMP + ADP where Thd is deoxythymidine, ATP is (energy-rich) adenosine 5’-triphosphate, TMP is deoxythymidine 5’-phosphate and ADP is adenosine 5’-diphosphate. Thymidine kinases have a key function in the synthesis of DNA and thereby in cell division, as they are part of the unique reaction chain to introduce deoxythymidine into the DNA...