There is a high production of this enzyme in fast‐proliferating neoplastic cells, and increased serum activity is caused by the leakage of TK1 after their death (Hallek et al., 1992); thus, the TK1 activity in serum correlates with the proliferative activity of the tumour (Hallek et al., 1992). The gene discussed is TK1; the disease is neoplasm.