Thymidine kinase expression. A marker for malignant cells.

The expression of thymidine kinase--an enzyme of the DNA precursor pathway--is strictly regulated during the normal cellular cycle, but is much higher and permanently expressed in malignant growing cells. Using a new cytofluorometric assay for thymidine kinase in single cells, we were able to discriminate between normal growing cells and virally transformed cells or lines derived from tumours. In material (blood and bone marrow) taken from leukaemia patients, we identified the leukaemic cells in a surplus of normal leucocytes. From cell cultures representing a tumour progression model, only the later, and malignant, stages showed enhanced fluorescence, whereas benign tumour cells looked normal.