Determination of serum thymidine kinase 1 (TK-1) activity is used as a tumour marker in both human and veterinary medicine. TK-1 is an intracellular enzyme involved in a salvage pathway of DNA synthesis. The expression of TK-1 is cell cycle dependent and the activity increases markedly after the G1 to S transition in the cell cycle and then declines rapidly in G2. The pronounced proliferative activity in tumour cells result in a higher TK-1 enzyme activity within the cell. The fact that the increased TK-1 activity can be measured in serum makes TK-1 a useful marker in diagnostics of neoplastic diseases and it provides information regarding prognosis and treatment effectiveness. In human medicine a radioactive based test is used to determine the TK-1 serum activity.