Thymidine kinase is an enzyme, a phosphotransferase (a kinase): 2'-deoxythymidine kinase, ATP-thymidine 5'-phosphotransferase, EC 2.7.1.21 that catalyzes the reaction: Thd + ATP → TMP + ADP where Thd is (deoxy)thymidine, ATP is adenosine 5’-triphosphate, TMP is (deoxy)thymidine 5’-phosphate and ADP is adenosine 5’-diphosphate. In clinical chemistry it has been suggested as a proliferation marker for prognosis, verification of diagnosis, control of treatment (particularly as a companion diagnostic) and follow-up of malignant disease. It is used mainly in relation to hematological malignancies but the developments of more sensitive assays have stimulated investigations for its use in relation to solid tumors.
