Using a cytofluorometric assay, we studied the time course of thymidine kinase activity during the cell-cycle in logarithmically growing cells. Two different patterns were found. Many cells, including all strains of normal diploid fibroblasts, exhibited a transient stimulation of enzyme activity during early S-phase. On the contrary, virally transformed cells or lines derived from tumors also increased enzyme activity during G1/S, but kept this level until mitosis. In the latter case, the absolute enzyme activity was significantly higher during all phases of the cell-cycle than in normal cells. Beside its importance for the understanding of the transformed state, this phenomenon has obvious benefits for tumor diagnosis.

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